This study represents the largest epidemiologic investigation of humans exposed to the radionuclide 210Po. In a cohort of 4402 white males employed by the Mound Facility, during the period when Po operations were conducted (1944-1972), no excess mortality was observed. Among workers initially hired during World War II, mortality was elevated, especially for deaths from all cancers, cancers of the lung, and cancers of the rectum. These adverse health events do not appear related to exposure to 210Po. Among workers monitored for 210Po, mortality was significantly less than expected, although more lung cancers were observed than expected. No significant dose-response trends were identified for all causes combined, all cancers combined, or for cause-specific cancers among the Po-monitored subcohort. Continued follow-up of this cohort is planned.