Levodopa (L-dopa) treatment of Parkinson's disease (PD) is associated with elevated homocysteine (Hcy). To examine the relationship between Hcy, methylenetetrahydrofolate reductase polymorphisms (MTHFR: 677C/T; 1298A/C), and B-vitamins in older PD patients and whether Hcy or MTHFR polymorphisms were associated with clinical measures. MTHFR polymorphisms, B-vitamin intake, and blood concentrations of Hcy, vitamin B12 and folate, and creatinine were determined and compared between groups (PD and controls). The relationship of Hcy to clinical measures was examined in PD. Among 51 patients [30M/21F, mean age (SD): 71.5 (4.7)] and 50 controls [29M/21F, 71.5 (4.8)], Hcy was higher in PD [13.6 (3.8); controls: 10.5 (2.5), P < 0.0005]. Hcy was associated with B-vitamin intake [F = 21.7, P < 0.0005], folate level (R = 0.31, P = 0.035), and the interaction of intake with MTHFR 677T (F = 5.2, P = 0.007), but not MTHFR 1298C genotype. Hcy did not correlate with global measures of cognition, mood, or parkinsonism in PD or with dyskinesias, fluctuations, or freezing. Higher vitamin B12 levels were associated with lower dyskinesia risk. Hcy was influenced by PD, MTHFR 677 genotype, and vitamin use, but not by the MTHFR 1298 genotype. There was no clear association with motor or cognitive measures, but dyskinesias were less likely with higher B12.