Malignant peripheral nerve sheath tumors (MPNST) are known to develop in patients with neurofibromatosis 1 (NF1), thus providing an excellent model for the study of multistep carcinogenesis in genetically predisposed individuals. To determine the sites of gene(s) involved in such a process, we have performed cytogenetic analysis on 10 tumors. The patients were five males and five females ranging in age from 15 to 77 years. Nine patients had NF1. Karyotypic analysis of these tumors exhibited complex clonal abnormalities of several chromosomes. Recurrent abnormalities (numerical as well as structural) of chromosomes 1, 11, 12, 14, 17, and 22 occurred in a substantial proportion of tumors studied. Although abnormalities of these chromosomes have been seen in a variety of other tumors, the aberrations of chromosomes 17 and 22 are of particular interest; chromosomes 17 and 22 carry the genes for NF1 and NF2, respectively. In addition to other clonal aberrations, six tumors had abnormalities of both chromosomes 17 and 22, while three tumors only had an abnormality of chromosome 17. In eight tumors a structural abnormality of chromosome 17 included deletion or a relative deficiency of 17p; in four of the tumors there was also either deletion or rearrangement of the NF1 locus at the cytogenetic level. One tumor had monosomy of chromosome 17. The abnormality of chromosome 22 was deletion of 22q11.2-->qter. This study suggests that the germline mutation in one of the copies accompanied by loss or inactivation of the second copy of the NF1 gene and tumor suppressor gene(s) on 17p and 22q may be associated with the neoplastic transformation; abnormalities of other chromosomes may be related to progression of MPNST. Although the role of the p53 gene in carcinogenesis is well documented in several tumor types, the role of the NF2 gene or other unidentified tumor suppressor gene(s) on chromosomes 22q, 1p, 11, 12, 14 remains to be seen.