Parkinson's disease (PD) patients have been reported to have lower bone mineral density (BMD) and higher fracture risk than individuals without PD. We assessed the association between hyperhomocysteinemia due to levodopa intake and BMD in PD patients. We measured serum homocysteine (Hcy) concentrations and BMD in the proximal femur and lumbar spine of PD patients aged 55 years or older (n = 95) and three age-/gender-matched control subjects (n = 285). The prevalence of osteoporosis was higher in both men (2.5-fold) and women (1.7-fold) with PD than in controls, and adjusted odds ratios for osteoporosis were 3.57 (95% confidence interval [CI], 1.25-10.20) for men and 2.54 for women (95% CI, 1.31-4.93) with PD. Serum Hcy concentrations were significantly higher in PD patients (median = 13.0 micromol/l) than controls (median = 11.5 micromol/l) (P = 0.005). Serum Hcy concentrations were independently associated with BMD values at all proximal femur sites in all subjects (P = 0.005 to 0.012). In PD patients, higher serum Hcy concentrations were independently associated with higher fracture risk (P = 0.029). PD patients taking higher doses of levodopa had significantly higher serum Hcy concentrations (P = 0.013), and greater levodopa intake was associated with lower BMD values in some areas (P = 0.008 to 0.029). In conclusion, these findings indicate that hyperhomocysteinemia due to levodopa intake may be one additional risk factor for osteoporosis and fracture in PD patients. Reducing Hcy may be a therapeutic modality for treating osteoporosis in PD patients taking levodopa.