Study of the Drug Ipilimumab for Metastatic Merkel Cell Carcinoma

* INCLUSION CRITERIA: * Diagnosis of Merkel cell carcinoma confirmed by the Laboratory of Pathology, NCI, or the participating institute s Department of Pathology. * Unresectable or metastatic Merkel cell carcinoma * Measurable disease as defined by lesions that are measured in at least one dimension \>20mm by CXR, as \> 10mm with CT scans, or \> 10mm with calibers by clinical exam. * Life expectancy greater than or equal to 6 months. * ECOG performance status of 0-2 * Willing to travel to the NIH or study sub-sites (MSKCC, UMich, or Penn) for follow-up visits. * Patients must have recovered from acute toxicities related to prior therapy or surgery. * Patients must receive at least one line of chemotherapy and achieve partial response(30% reduction in tumor burden) or better prior to enrollment. EXCEPTION: Patients with asymptomatic tumors showing no or minimal progression (\<20% tumor burden) within the last 2 months can be enrolled without prior chemotherapy. * Age greater than equal 18 years. Because no dosing or adverse event data are currently available on the use of ipilimumab in patients \<18 years of age, children are excluded from this study, but may be eligible for future pediatric trials. * Patients must have adequate hematological, hepatic, and renal laboratory values, as defined below: * leukocytes \>3,000/mcL * absolute neutrophil count \>1,500/mcL * platelets \>100,000/mcL -----total bilirubin within normal institutional limits (except subjects with Gilbert s Syndrome, who must have a total bilirubin less than 3.0 mg/dL) * AST(SGOT)/ALT(SGPT) \<2.5 times institutional upper limit of normal * creatinine \<2.0 times institutional upper limit of normal. * The effects of ipilimumab on the developing human fetus are unknown. For this reason and because ipilimumab was found to have teratogenic and abortifacient effects in animal studies, women of child-bearing potential and men must agree to use adequate contraception (hormonal or barrier method of birth control; abstinence) prior to study entry, for the duration of study participation, and for 4 months after the last injection of ipilimumab. Should a woman become pregnant or suspect she is pregnant while she or her partner is participating in this study, she should inform her treating physician immediately. * Ability of subject to understand and the willingness to sign a written informed consent document. EXCLUSION CRITERIA: Prior treatment with ipilimumab. -Patients who are immunocompromised as listed: * Human immunodeficiency virus infection, or Hepatitis B or C infection, due to the unknown effects of ipilimumab on viral replication and immune function and the potential for severe side effects. * Chronic administration (defined as daily or every other day for continued use \>14 days) of systemic corticosteroids (including steroid eye drops) or other immune suppressive drugs, within 14 days of the first planned dose of ipilimumab. Nasal, or inhaled steroid, and topical steroid creams for small body areas are not excluded. * Patients who have undergone allogeneic peripheral stem cell transplantation, or solid organ transplantation requiring immunosuppression Prior Splenectomy -Patients with history of, or active autoimmune disease that has required treatment, such as Addison's disease, autoimmune thyroiditis, Grave s disease, systemic lupus erythematous, rheumatoid arthritis, autoimmune hepatitis, systemic sclerosis (scleroderma and variants), autoimmune vasculitis, autoimmune neuropathies (such as Guillain-Barr(SqrRoot)(Copyright) syndrome), Goodpasture syndrome, ulcerative or hemorrhagic colitis, autoimmune hypophysitis/hypopituitarism, and autoimmune hemolytic anemia. EXCEPTIONS: Patients with a history of autoimmunity that has not required systemic immunosuppressive therapy or does not threaten vital organ function including CNS, heart, lungs, kidneys, skin, and GI tract will not be excluded. Patients with type 1 diabetes mellitus, or vitiligo, or endocrine deficiencies such as hypothyroidism will not be excluded if the condition is well controlled. * Other active malignancies within the past 12 months. EXCEPTIONS: Patients with adequately treated basal or squamous cell carcinoma of the skin or in situ carcinoma of the cervix or bladder will not be excluded. Patients with low or intermediate-risk CLL (Rai stage 0-II, Binet stage A or B) without progressive or symptomatic disease, who are being monitored without therapy will not be excluded. * Clinically significant (i.e., active) cardiovascular disease: cerebral vascular accident/stroke \< 6 months prior to enrollment, myocardial infarction \< 6 months prior to enrollment, unstable angina, or congestive heart failure ( (Bullet) NYHA III). * Pulmonary disease which, in the opinion of the investigator, may impair the patient's respiratory tolerance to the study drug (e.g., interstitial lung disease, severe chronic obstructive pulmonary disease). * Presence of serious or life-threatening intercurrent medical illness. * Patients with symptomatic or progressive (progression within the last 3 months) brain metastases are excluded from this clinical trial because of their poor prognosis and because they often develop progressive neurologic dysfunction that would confound the evaluation of neurologic and other adverse events. * Patients who have had a history of acute diverticulitis, intra-abdominal abscess, GI obstruction and abdominal carcinomatosis which are known risk factors for bowel perforation. * Pregnant women are excluded from this study because, based on animal studies, ipilimumab has the potential for teratogenic or abortifacient effects. Because there is an unknown but potential risk for adverse events in nursing infants secondary to treatment of the mother with ipilimumab, breastfeeding should be discontinued if the mother is treated with ipilimumab. * Any condition which, in the opinion of the investigator, would prevent full participation in this trial (including the long-term follow-up), or would interfere with the evaluation of the trial endpoints. * Receipt of chemotherapy or an investigational agent within 21 days (or 60 days for an antibody-based therapy) of the first planned dose of study drugs.