Forty-six cases of malignant pleural mesothelioma were analyzed for histologic subtype, DNA content, and cell cycle characteristics. Sixty-five percent of cases were diploid in DNA content, with intermediate to low proliferative rates. Thirty-one nonmesothelial malignant neoplasms of the lung, of histologic types most easily confused with malignant mesothelioma, were also examined. In contrast to the mesotheliomas, 85% of these nonmesothelial malignant neoplasms of the lung were aneuploid; the aneuploid neoplasms exhibited higher mean proliferative rates (S = 10.6%) than diploid nonmesothelial neoplasms of the lung (S less than 6%). Unlike most malignant neoplasms, mesotheliomas most often display diploid DNA contents and low proliferative rates despite their clinically aggressive behavior.