Recently a high radon concentration was detected in the underground coal mine of Figueira, located in the south of Brazil. This coal mine has been operating since 1942 without taking cognizance of the high radon environment. In order to assess possible radon-related health effects on the workers, a retrospective (1979-2002) mortality study of 2,856 Brazilian coal miners was conducted, with 2,024 underground workers potentially exposed to radon daughters. Standard mortality ratio (SMR) analysis hints at lower mortality from all causes for both underground (SMR = 88, 95% CI = 78-98) and surface workers (SMR = 96, 95% CI = 80-114). A high statistically significant SMR for lung cancer mortality was observed only in the underground miners (SMR = 173, 95% CI = 102-292), with a statistically significant trend reflecting the duration of underground work. High statistically significant SMRs were observed for pneumonia as a cause of death between both surface (SMR = 304, 95% CI = 126-730) and underground miners (SMR = 253, 95% CI = 140-457). Because mortality from smoking-related cancers other than lung cancer was not found elevated in underground workers and because diesel equipments were not used in this mine, it can be concluded that the enhanced lung cancer mortality observed for underground miners is associated with exposure to radon and radon daughters, rather than other confounding risk factors.