Experimental studies have demonstrated that certain types of commercially produced polychlorinated biphenyls (PCBs) are carcinogenic. Data in humans are still controversial. This study was undertaken in order to determine possible long-term effects, particularly cancer, in workers engaged in the manufacture of capacitors impregnated with PCBs in a plant operating since 1946. All workers employed for at least 1 week between 1946 and 1978 were admitted to the study (544 males and 1,556 females), and their mortality was examined for the period 1946-1982. Data on environmental contamination, workers' PCBs intake, and health effects (chloracne) were available, which documented the general exposure conditions in the plant. Vital status was ascertained for over 99% of the subjects, and death certificates were obtained for all deceased persons. Expected deaths were calculated using two sets of mortality rates, national and local. Among male workers, cancer deaths (14 obs.) were significantly increased as were deaths owing to cancer of the gastrointestinal (GI) tract (6 obs.). Also, mortality from hematologic neoplasms (3 obs.) and lung cancer (3 obs.) was higher than expected; however, the excess was statistically not significant. Female workers exhibited an overall mortality that was significantly increased above expectations. Cancer deaths (12 obs.) and hematologic neoplasms (4 obs.) were significantly higher than expected when compared with the local population. Interpretation of the results is limited by the small number of deaths; however, the point of interest is the consistency of these results with previous experimental and epidemiologic studies, which indicated the GI tract and lymphatic and hemopoietic tissue as the most probable target sites of the PCBs carcinogenic activity.