Monoamine oxidase B (MAO-B) is an enzyme that has relevance for Parkinson disease (PD) because of its roles in catabolizing dopamine and potentially activating exogenous neurotoxicants. A polymorphism of the gene encoding MAO-B has been identified as a single base change (A or G) in intron 13 of the X chromosome. The A allele was previously associated with an approximately twofold risk of PD. The present study compared A and G allele frequencies between newly diagnosed idiopathic PD cases and a control group free of neurodegenerative diseases. All study subjects were Caucasian. Cases were 37 men and 25 women, age 37-80 years; controls were 50 men and 29 women, age 45-82 years. MAO-B genotype was determined by the allele-specific polymerase chain reaction on DNA extracted from peripheral lymphocytes. In complete contrast to previous studies, elevated risks were detected with the G allele. The age-adjusted odds ratio for the G allele in males was 1.87 ((95% confidence interval) 0.78-4.47). Among females the age-adjusted odds ratios were 5.00 ((95% confidence interval) 1.13-22.1) for the GA genotype and 5.60 ((95% confidence interval) 1.01-30.9) for the GG genotype. These findings, although of limited statistical precision, suggest that the G allele of this MAO-B polymorphism may relate to PD risk.