An epidemiological cohort study has been carried out to demonstrate any excess cancer mortality in a group of French uranium miners who worked underground for at least 2 years. The latency period of cancer associated with this type of professional exposure was long (10 to 20 years). The first study includes miners who began underground mining between 1946-1972. The individual monthly exposure to radon and its decay products has been recorded since 1956. The exposure of the first 10 years had to be estimated retrospectively for each miner regarding the working conditions. Problems associated with these two different exposure recordings are discussed. The results revealed an excess of lung cancer mortality for miners who had begun underground mining during the years 1946-1956.