Mesothelioma is a rare neoplasm, the occurrence of which has been clearly related to asbestos exposure. Data recently collected by population based cancer registries in Washington, Hawaii, New Mexico, Connecticut, Michigan, Utah, Louisiana and Iowa were obtained for the determination of mesothelioma incidence rates. These registries represent a total population of over 16 million persons. Crude incidence rates were found to range from 3.0 to 7.1 per million per year. Age specific incidence rates showed a steady increase from the third through the eighth decade, with a more rapid increase for males than females. Sex specific incidence rates, age adjusted to the 1970 U.S. population, were found to range from 4.4 to 11.1 per million per year for males and from 1.2 to 3.8 per million per year for females. The highest rates for both males and females were found in the New Orleans area of Louisiana and the Puget Sound area of Washington state, both with significant shipbuilding activity.