BACKGROUND: The role of genetics in early-onset Parkinson's disease has been established, but whether there is a genetic contribution to the more common, late-onset form remains uncertain. METHODS: We reviewed the medical records and confirmed the diagnosis of Parkinson's disease in 772 living and deceased patients in whom the disease had been diagnosed during the previous 50 years in Iceland. With the use of an extensive computerized data base containing genealogic information on 610,920 people in Iceland during the past 11 centuries, several analyses were conducted to determine whether the patients were more related to each other than random members of the population (control subjects). RESULTS: Patients with Parkinson's disease, including a subgroup of 560 patients with late-onset disease (onset at >50 years of age), were significantly more related to each other than were subjects in matched groups of controls, and this relatedness extended beyond the nuclear family. The risk ratio for Parkinson's disease was 6.7 (95 percent confidence interval, 4.3 to 9.6) for siblings, 3.2 (95 percent confidence interval, 1.2 to 7.8) for offspring, and 2.7 (95 percent confidence interval, 1.6 to 3.9) for nephews and nieces of patients with late-onset Parkinson's disease. CONCLUSIONS: Late-onset Parkinson's disease has a genetic component as well as an environmental component.