Genetic factors seem to be important in the pathogenesis of Parkinson's disease (PD). It is however, still controversial whether these factors also are reflected in a familial aggregation of PD. The aim of this study was to investigate the frequency of PD patients with a positive PD family history compared with two control groups. The included 245 PD patients were examined by neurologists and information was obtained through a semi-structured interview. The patients and the control groups were examined for the frequency of PD and dementia in their families. The 245 patients with PD were included in this study. A positive PD-family history could be obtained in 53 (21.6%) patients. The frequency was three- and four-fold increased as compared with the control groups (P < 0.001). Age at onset of disease was not different among patients with and without PD in the family. The frequency of dementia did not differ in the family of individuals with and without PD (P > 0.1). As a conclusion our study of PD in a community based population supports previous reports of a three- to fourfold increased risk for PD in the families of patients with the disease. Our results indicate that the familial aggregation of the disease is independent of the age of the proband.