The clinical features of familial Parkinson's disease (PD) were investigated by examining the families of 20 British probands who were selected on the basis of having clinically typical PD and at least one affected relative. Forty-nine secondary cases were identified. These subjects were clinically indistinguishable from sporadic cases of idiopathic PD. If it is assumed that familial PD has a genetic basis, pedigree and segregation analysis suggested autosomal dominant inheritance of a gene or genes with reduced penetrance as the most likely explanation. The data did not support the possibilities of either mitochondrial or polygenic inheritance, although the latter cannot be excluded. The role of genetic factors in sporadic cases of PD remains unclear.