Reports on familial occurrence of essential thrombocythemia (ET) are scanty. Many clinical and hematological aspects of familial ET have not been clarified yet. We studied 16 family members in four successive generations. By laboratory tests and bone marrow examination they were divided into a non-thrombocythemia group (n = 5) and into ET patients (n = 11). Five ET patients were asymptomatic, three patients had both vaso-occlusive and hemorrhagic symptoms, and three patients only vaso-occlusive symptoms. The platelet count ranged from 500 to 1700 x 10(9)/l. Symptoms correlated with age but not with platelet count. ADP-induced platelet aggregation distinguished best between patients and non-ET subjects. Four patients and four non-ET subjects had factor VIII:C or von Willebrand factor antigen abnormalities; all but one had blood group O. These abnormalities were not due to inherited von Willebrand's disease according to haplotype analysis. Two patients and three non-ET subjects had a bleeding diathesis. One of these two patients and all three non-ET subjects had a decreased factor VIII:C or vWF:Ag. No chromosome abnormalities were found. In conclusion, familial ET has a relatively benign course with clinical manifestations similar to nonfamilial cases, and it is probably transmitted by an autosomal dominant mode of inheritance.