Cytogenetic study was made of mesothelioma cells from 14 patients. Metaphases were obtained from 12 tumors and revealed aneuploidy and clonal abnormalities in 9 specimens. In the two remaining cases, no metaphases were obtained. The cytogenetic abnormalities were complex, and up to 12 marker chromosomes were observed in the tumors. Rearrangements of chromosomes #1, #2, #3, #6, #9, #11, #17, and #22 were most frequently observed. Chromosome markers involved diverse bands, including several that are loci of oncogenes, fragile sites, and nonrandom rearrangements in other types of cancer. This study shows that the karyotypes of malignant mesothelioma can be analyzed by standard cytogenetic techniques. Additional studies of untreated mesothelioma may help to distinguish primary cytogenetic changes from effects of prior therapy in some of our patients.