The role of heredity in Parkinson's disease (PD) is controversial. We report a pair of monozygotic twins (confirmed by DNA fingerprints) concordant for PD. Their disease began when they were 62 and 63 years old. Both patients presented with left-side bradykinesia. One of the twins had a long history of depression. Both patients had typical manifestations of PD, which were responsive to dopaminergic therapy. The similar age of onset along with the similar clinical characteristics of these twins suggests that hereditary or genetic susceptibility may be important in the etiology of PD.