An unexpectedly large number of peritoneal mesotheliomas among women was reported to the Danish Cancer Registry during the years 1960 through 1985. In a case-control study, we tested whether this was due to diagnostic misclassification or to direct exposure of the peritoneal membranes to talc during abdominal surgery. Tissue specimens were available from 96 reported peritoneal mesotheliomas; 35 cases (37%) were regarded as verified mesotheliomas, and 33 (34%) as possible mesotheliomas. Information on any intraabdominal surgery was obtained from the hospital records of these 68 cases and for 206 controls with a cancer of the uterine corpus or pancreas. No association was seen between peritoneal mesotheliomas and abdominal surgery performed more than 25 years prior to cancer diagnosis (odds ratio, 0.83; 95% confidence interval, 0.35-1.94). CONCLUSION. Misclassification of other cancers was the reason for the observed excess of peritoneal mesotheliomas.