Four cases of thrombocytosis in three successive generations of a family are described. High peripheral platelet count was found incidentally in the proband with cutaneous malignant lymphoma. Bone marrow examination showed megakaryocytic hyperplasia. Neither Philadelphia chromosome nor chimaeric bcr/abl junction was detected in marrow cells. In this family, thrombocytosis was thought to be transmitted by an autosomal dominant mode of inheritance.