We performed a case-control study on 100 patients with Parkinson's disease, their spouses and the same number of sex- and age-matched neurological controls to clarify if family history of Parkinson's disease or essential tremor may increase the risk for the disease. We included in the study 68 male and 32 female parkinsonian patients with a mean age +/- SD of 62.0 +/- 9.9 years and a mean disease duration of 7.5 +/- 5.7. The odds ratio for familial Parkinson's disease was 13.4 (95% confidence limits = 6.5-27.7) and for familial essential tremor 3.1 (95% confidence limits = 1.5-6.3). We also reviewed the genetic features of 122 parkinsonian patients with at least one affected relative. The presence of secondary cases among both first-degree (n = 83) and less close relatives (n = 72) suggests that sharing environmental factors does not explain the familial aggregation of the disease. Secondary cases were significantly more frequent in the paternal than in the maternal line (70 vs. 39). The presence of secondary cases among both siblings (46) and parents (37) and the unilateral distribution of ancestral secondary cases suggest an autosomal dominant inheritance with incomplete penetrance.