Cytogenetic analysis was performed on 11 peripheral nerve sheath tumors of soft tissue from 10 patients. They include 6 benign and 5 malignant schwannomas. Five cases which include two benign, one cellular and two malignant schwannomas had a known association with a nerve, but only one patient with malignant schwannoma has clinically documented neurofibromatosis type I. All the patients had a normal diploid constitutional karyotype. Two cases of cellular schwannoma were analyzed by routine cytogenetic analysis and fluorescence in situ hybridization (FISH). One tumor was karyotyped as 45, XX,-13,-22 +mar; and the other case had a 45,X,-Y,t(1;17) (p12;q11.2) karyotype. In the latter, the breakpoint in 17q occurred below the centromere and is at or in the region of the Neurofibromatosis Type 1 (NF1) gene. Four benign tumors had a normal diploid karyotype. One hypodiploid malignant schwannoma with myxoid features demonstrated monosomy of chromosomes 17 and 22 by FISH analysis. The rest of the malignant schwannomas showed a wide range of numerical and structural aberrations, with frequent loss of 22q and gains of chromosomes 2 and 7. Loss of a sex chromosome was observed in cellular as well as malignant schwannomas. Regional karyotypic evolution was noted in one malignant schwannoma. Cytogenetic analysis may prove to be useful in identifying tumors, such as cellular schwannomas, which, because of their histologic features may be inadvertently categorized as malignant. Simultaneous involvement of NF1 and NF2 genes, which are located on chromosomes 17q and 22q, respectively, should be investigated at a molecular level in both benign and malignant tumors of peripheral nerves.