Vitamin B12 Status and Response to Vitamin B12 Supplementation in Patients With Parkinson's Disease

Inclusion Criteria: * Patients seen in Emory Healthcare System at least once between 1/1/2002 and 05/31/2006 with idiopathic Parkinson's disease (defined as): * \>2 of cardinal signs/symptoms of Parkinson's disease (bradykinesia, tremor, rigidity) * \>50% improvement of symptoms when dopaminergic therapy started * Cohabiting spousal caregivers (without Parkinson's disease) of patients enrolled in study are also eligible (but not required) to participate * Coexisting depression, if present, is stable * Normal thyroid screen (done at Screening Visit or at Emory Healthcare System within 3 months of Screening Visit) * Males must have normal fasting testosterone levels (done at Screening visit or within 6 months of Screening Visit) Exclusion Criteria: * Unstable medical conditions (terminal cancer, angina, etc) which could interfere with study assessments or put patient at risk for not completing the assessments. * Dopaminergic-unresponsive parkinsonism (i.e., patient has suspected multiple systems atrophy (MSA), vascular or another form of parkinsonism) * Prior history of vitamin B12 deficiency or taking vitamin B12 supplements or injections (single multivitamin supplements are not an exclusion criteria) * Untreated testosterone or thyroid abnormality