In the present paper we explore in some detail the hypothesis that the presence of familial aggregations in 10-15% of Parkinson's disease cases is due in great part to the existence of well-defined familial subsets, rather than to chance occurrences. We describe the clinical and genetic characteristics of the two main subsets: "Essential tremor-related Parkinsonism" and the "Familial akineto-rigid Syndrome" previously identified. The former type of Parkinsonism is associated at random, but with increased frequency, to an autosomal dominant disorder, usually essential tremor but occasionally OPCA. Two possible susceptibility factors were uncovered in this entity: an increased incidence of familial hyperthyroidism (augmentor factor) and a decreased incidence of the generally frequent HLA Haplotypes A1B8 or A2B5 (Protective factors). The other presentation, the "familial akineto-rigid syndrome", appears to be a definite disease entity with an autosomal recessive mode of inheritance (normal parents, increased incidence of identical parkinsonism in sibs, increased consanguinity rate in parents). This newly defined disorder deserves much further genetic and biochemical analysis.