Idiopathic Parkinson's disease (PD) is an age dependent, neurodegenerative disorder and is predominantly a sporadic disease. A minority of patients has a positive family history for PD and the majority of those families exhibit a complex mode of inheritance. The monoamine oxidases A and B (MAO-A and -B) genes, which are involved in serotonin and dopamine metabolism, are possible candidate genes for susceptibility to PD. Previous association studies of MAO-A and -B in PD have been inconclusive. To determine the role of MAO-A and -B in the development of PD, we screened a sample of 96 patients with familial PD, 164 with sporadic PD, and 180 matched normal controls with dinucleotide repeat markers in these genes. MAO-A and -B gene polymorphisms were strongly associated with total PD (p < 0.00001), familial PD (p < 0.00001), and sporadic PD (p < 0.00001). There were no significant differences between familial or sporadic PD with age of onset younger than 50 years compared to those with age of onset older than 51 years for both MAO-A and -B genes. There was no linkage disequilibrium between these genes in male PD and control groups. The frequency of common haplotypes from MAO-A and -B was different in PD and control group (p = 0.02). Our data indicate that MAO-A and -B may play a role in susceptibility to PD in our sample.