A girl with neurofibromatosis type 1 showed in infancy extreme thrombocytosis with platelet counts exceeding 2000 x 10(9)/L. Platelet counts remained elevated the first 4 years of life. A large deletion located at chromosome 17q11 was observed. The transient thrombocythemia may relate to a regulator of megakaryocytopoiesis located in the vicinity of the NF1 gene or to the excessive risk of chronic myelomonocytic leukemia in neurofibromatosis.