AbstractTo 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 Parkin