The histopathological characteristics of malignant mesothelioma occurring in asbestos insulation workers were examined. A total of 278 definite or suspected cases of malignant mesothelioma was identified in an ongoing prospective mortality survey of 17800 asbestos insulation workers in the United States and Canada. The histopathological features of the tumors and their relationship to factors such as severity of pulmonary fibrosis, age at death, years from first asbestos (1332214) exposure to time of death (latency), and cigarette smoking were evaluated. A total of 234 cases were classified as definite mesothelioma and 44 as probable mesothelioma. The primary tumor sites were: peritoneal, 160 cases; pleural, 97 cases; other, 21 cases; pleural and peritoneal, 18 cases; pleural, peritoneal, and pericardial, two cases; and pleural and pericardial, one case. Cell types were classified as epithelial in 64% of the cases, biphasic 27%, and fibrous 9%. The distribution of the cell types among pleural and peritoneal mesotheliomas was similar. A total of 250 cases were 50 years or older at time of death. None were 39 years or younger. Approximately 18.3% of the cases had latency periods of 20 to 29 years, 47.3% had latency periods of 30 to 39 years, 21.2% had latency periods of 40 to 49 years, and 12.8% had latency periods of 50 years or longer. None had latency periods shorter than 9 years and one case had a latency period of 10 to 19 years. A total of 137 cases of pulmonary asbestosis were also seen. Of these, 131 were diffuse interstitial fibrosis accompanied by ferruginous bodies. The degree of diffuse interstitial fibrosis was moderate or severe in 100 cases. Of the 185 cases in which smoking histories were available, 144 were current cigarette smokers at the time of death. Seventeen were pipe or cigar smokers. Twenty were nonsmokers. The authors conclude that the incidence of peritoneal mesothelioma in asbestos insulation workers is higher than that of pleural mesothelioma. The reasons for the high incidence of peritoneal mesothelioma are not known. The relative proportion of nonsmokers among the mesothelioma cases is higher than that seen in lung cancer cases in the same cohort.