Malignant mesotheliomas have assumed an importance in the medical and lay literature out of proportion to their incidence in the American population, chiefly due to the known association of pleural mesothelioma with asbestos exposure. Data from the Connecticut tumor registry suggest that this tumor is increasing in incidence. Based on exposures between 1940 and approximately 1970, when industrial precautions were first instituted, epidemiologists estimate that the number of new cases of mesothelioma will peak sometime in the 1990s and that mesothelioma will thereafter become less common. These models generally assume that asbestos exposures after 1970 will be insignificant. While industrial levels that are legal today will almost certainly prevent the development of severe asbestosis in most workers, unfortunately a threshold of exposure below which there is no risk of mesothelioma has not been documented. Asbestos continues to be used in floor and ceiling tiles, in automobile brake linings, and in a variety of other products. At the present time, construction workers who maintain or remove asbestos constitute one of a number of groups with continued exposure. The diagnosis of a malignant pleural mesothelioma is not difficult provided that the physicians caring for such a patient consider mesothelioma in the differential diagnosis. Patients present with chest pain or shortness of breath, or both, and the initial chest x-ray film most often reveals a large unilateral pleural effusion. The tumor characteristically remains localized until late in its course, and thus extensive workup at the time of diagnosis is seldom required. Generally, a large piece of tissue obtained via an open biopsy is required for adequate histologic diagnosis. Investigational approaches include taking numerous needle biopsies with samples sent for electron microscopy as well as for immunoperoxidase staining for keratin and CEA. The treatment of this disease remains unsatisfactory. Occasional patients have remained disease-free for periods in excess of 5 years after intensive treatment, however. The conclusion that mesothelioma is untreatable is clearly untenable since palliation and a response rate of 30 per cent to various chemotherapeutic regimens have been reported by a number of investigators. While many authors have advocated supportive care alone because "current treatments have not demonstrated increased survival," we believe patients with mesothelioma should be offered investigational therapy.(ABSTRACT TRUNCATED AT 400 WORDS)