Parkinson's disease in a nationwide twin cohort.

The Finnish Twin Cohort includes all Finnish same-sexed twins born before 1958 and alive in 1967; the number of individuals alive in 1975 was 33,247. We performed a search for cases with Parkinson's disease among this cohort by linking the Twin Cohort Register with the Finnish Hospital Discharge Register and the Finnish Sickness Insurance Register. We ascertained altogether 42 cases of Parkinson's disease occurring in 41 twin including 18 monozygotic pairs, 14 dizygotic pairs, and nine pairs of undetermined zygosity. Only one dizygotic pair was concordant for Parkinson's disease; all other pairs were discordant. In 1981, the expected number of cases among the Twin Cohort, calculated according to the age- and sex-specific prevalence rates of Parkinson's disease in Finland, was 33. At the same time, the observed number of patients alive was 35. This study, further substantiating the low concordance for Parkinson's disease in monozygotic as well as in dizygotic twins and indicating that the prevalence of Parkinson's disease in twins compares with the prevalence in the general population, suggests that Parkinson's disease is an acquired disease not caused by a hereditary process.