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Familial aggregation of Parkinson's disease in a Finnish population was investigated. A family history was obtained on 268 patients with Parkinson's disease and 210 controls ascertained from the population of the province of northern Ostrobothnia, Finland. Ten per cent of the probands reported an affected first degree relative, whereas the corresponding frequency was 3.8 per cent in the controls (p=0.01). The relative risk of Parkinson's disease among the first degree relatives of the patients with Parkinson's disease was 2.9 (95 % confidence interval 1.3-6.4) and the cumulative incidence of Parkinson's disease by the age of 90 years was 3.3-fold higher among the first degree relatives of the patients than those of the controls. The crude segregation ratio was 0.27 for the siblings and 0.17 for the parents suggesting that recessive inheritance may be more common than dominant inheritance among Finnish patients with Parkinson's disease.