Parkinson's disease in 65 pairs of twins and in a set of quadruplets.

Among 43 monozygotic (MZ) and 19 dizygotic (DZ) pairs in which an index case had definite Parkinson's disease (PD), only one MZ pair was definitely concordant for PD. When pairs with questionable clinical features were included, 4 of 48 MZ and 1 of 19 DZ pairs were concordant. The frequency of PD in MZ cotwins of index cases with PD was similar to that expected in an unrelated control group matched for age and sex. Although we were unable to identify a single environmental agent, we conclude that the major factors in the etiology of PD are nongenetic.