To clarify the role of heredity and of some environmental risk factors in the etiology of idopathic Parkinson's disease, we performed a case-control study in two regions of southern Italy, Campania and Molise. We selected two controls for each parkinsonian patient, the patient's spouse and a sex- and age-matched neurological control. One hundred sixteen consecutive outpatients with Parkinson's disease (77 men, 39 women; mean age ± SD = 62.5 ± 9.9) and the same number of spouses and neurological controls were interviewed about five environmental risk factors (cigarette smoking, well-water drinking, head trauma with loss of consciousness, strict diets, general anesthesia) and two genetic risk factors (family history of Parkinson's disease or of essential tremor). Well-water drinking and family history of Parkinson's disease or essential tremor showed a positive association with Parkinson's disease; smoking showed a negative association. The most relevant risk factor was history of familial Parkinson's disease (odds ratio = 14.6; 95% confidence interval = 7.2 − 29.6); 33% of our patients had at least one affected relative. We also showed a unilateral distribution of ancestral secondary cases on the paternal or on the maternal side, which suggests a dominant inheritance. Clinical and epidemiologic features of cases with familial Parkinson's disease showed no peculiarity. The study suggests a strong role of the genetic factors in the etiology of Parkinson's disease.