Primary or essential thrombocythemia is rarely observed in childhood, and familial occurrence has been reported only once. In this study, essential thrombocythemia is documented in five members of both sexes from two to 62 years of age in three successive generations. The propositus had a persistent elevation of the platelet count, splenomegaly, a normal hemoglobin level, a normal white blood cell count, and abnormal platelet aggregation. Platelet arachidonic acid metabolites assayed by high-performance liquid chromatography and serum thrombopoietin levels were normal. Megakaryocytes were increased in number and size. Both mature and early immature megakaryocytes, but no atypical megakaryocytes, were identified by surface immunofluorescence. Bone marrow cultures showed normal myeloid and erythroid colony formation, and chromosome studies revealed a normal female karyotype. These findings support the concept that familial essential thrombocythemia is a myeloproliferative disorder that is transmitted by an autosomal dominant mode of inheritance, and that untreated young women and children with essential thrombocythemia have long survival.