The occurrence of polycythemia vera in a father, mother, and two sons is reported. Thirteen kindreds with familial polycythemia vera in 31 members are reviewed. Comprehensive records were available for all four patients as well as other family members, since all were diagnosed and treated at the author's institution over a period of nearly 50 years. The mean age at diagnosis, sex predominance, symptoms, and incidence of chromosomal abnormalities, leukocytosis, thrombocytosis, and elevated leukocyte alkaline phosphatase levels were similar to those of nonfamilial cases. The mean RBC volume at diagnosis and the incidence of splenomegaly appear to be higher in familial than nonfamilial cases. The mode of inheritance is unclear, but genetic factors may be involved in the pathogenesis of this myeloproliferative disorder.