The authors studied Parkinson disease (PD) in natives of Crete, a population sharing a common genetic and cultural background. Analysis of data from 247 index PD cases revealed a hereditary component for PD (OR 3.37; p < 0.001 for a first-degree relative of a PD patient). In 12 bilineal families (PD originating from both parental sides) identified, a high proportion (43%) of bilineal members had PD, but only 5.7% of their offspring were affected. These data suggest a recessive or an oligogenic inheritance for PD.