Increasing evidence suggests that elevated levels of homocysteine (Hcy) and methylmalonate (MMA) may be involved in the pathogenesis of neurodegenerative diseases. The urine levels of MMA and serum levels of Hcy as well as folic acid and vitamin B(12) were measured in patients suffering from the distinct neurodegenerative diseases progressive supranuclear palsy (PSP), amyotrophic lateral sclerosis (ALS) and Parkinson's disease (PD), and compared to age- and gender-matched control subjects. We found significantly elevated concentrations of Hcy (PD 15.1, PSP 15.8, ALS 13.9, control 11.2 micromol/l) and MMA (PD 3.7, PSP 3.1, ALS 3.7, control 1.8 mg/g) in all patient groups in comparison with controls. Levels of Hcy and MMA did not differ significantly between the neurodegenerative diseases. Our findings might imply that Hcy and MMA are released as a consequence of neurodegeneration regardless of the underlying cause and serve as surrogate markers of neurodegeneration. Alternatively they might be directly implicated in the pathogenesis of these diseases. Since elevated levels of both Hcy and MMA are neurotoxic, further studies might investigate the effect of vitamin therapy on disease progression.