Familial essential thrombocythemia.

We report three siblings (2 F, 1 M) aged 9, 12, and 15 years with a thrombocytosis, above 650 X 10(9)/l for at least 18 months and a leukocyte alkaline phosphatase score below 37. The diagnosis of essential thrombocythemia was based upon the exclusion of a reactive thrombocytosis or another myeloproliferative disease. Cytogenetic examination of marrow cells was normal.