A girl with neurofibromatosis type 1 showed in infancy extreme thrombo
cytosis 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 r
elate to a regulator of mega-karyocytopoiesis located in the vicinity
of the NF1 gene or to the excessive risk of chronic myelomonocytic leu
kemia in neurofibromatosis.