We studied chromosomes of BM cells from four neurofibromatosis (NF) patients with leukemia. One patient had a normal diploid karyotype in the chronic phase of juvenile chronic myelogenous leukemia (JCML). When the the leukemia evolved into the accelerated phase, she had cells with 46,XX,-7,+der(7)t(3;7)(q21;p22); the abnormalities resulted in a partial 7p deletion. In another patient with JCML, BM cells in the accelerated phase had 45,XY,-7. The abnormal cells with monosomy 7 disappeared from the BM after chemotherapy but reappeared later in the course. Another patient developed refractory anemia with excess of blasts in transformation (RAEB-T) and had cells with 46,XX,-6,+r(6)(p23?q21?); the abnormalities resulted in partial 6p and 6q deletions. The other patient with ANLL had cells with 45,XX,-7. Our findings and review of data on nine other patients suggest that BM cells of NF patients with JCML in chronic phase have no microscopically detectable chromosome changes and that cells with chromosomal deletion emerge when JCML evolve into the accelerated or blast phase. Thus, deletion of the whole or part of certain chromosomes, such as chromosomes 6, 7, etc., may be an important step towards the evolution of JCML cells or the development of de novo acute leukemias in NF patients.