Use of Sildenafil (Viagra) to Alter Fatigue, Functional Status and Impaired Cerebral Blood Flow in Patients With CFS

Inclusion Criteria: * Patients meeting the CDC definition of CFS. * All races, ethnicities, socio-economic status (SES), and gender * Age greater than 18 (because of concerns about radioactivity, we and the Cedars-Sinai and Harbor-UCLA IRBs have decided not to enroll subjects below the age of 18). * Age less than 50. Because of concern of sildenafil exacerbating coronary artery disease, we will only enroll patients younger than 50. * Able to provide informed consent. * Willingness to be off all medicines and supplements for 3 weeks prior to the study. * Patients with psychiatric disorders (see below) will be included, if they could be off their medications, and if their psychiatric diagnosis clearly occurred after their fatigue symptoms began. * Patients with concurrent fibromyalgia will be allowed to participate if the meet diagnostic criteria for CFS. Exclusion Criteria: * Disabilities that would prevent them from participating in the study. * Current use of prescription medicines (starting at 3 weeks prior to the study) and supplements (starting at 1 weeks prior to the study) except acetaminophen or aspirin. This includes herbal supplements and vitamins. * Existing medical illnesses, such as heart disease, hypertension, cancer, rheumatological diseases, endocrinopathies or hormone replacement therapy, seizure disorders, severe obesity (BMI \> 32 kg/m2), * Severe psychiatric disorders including bipolar disorder, schizophrenia, dementia and previous or current diagnosis of alcohol or substance abuse within the past year. Patients with depression of such severity as to warrant treatment with anti-depressants will be excluded. * Current abuse of illicit drugs or heavy ethanol use. * Pregnant women will be excluded because of radioactivity exposure from the SPECT scans. * Abnormal EKG * Abnormal CBC, blood chemistries, thyroid function tests, and HIV, ANA, RF and ESR tests.