The exposure of swimmers to chloroform (CHCl5) was investigated in indoor swimming pools of the Quebec City region along with the associated carcinogenic risk. Six training sessions involving 52 competition swimmers (11 to 20 yr old) were conducted in 3 different pools, while 12 adult leisure swimmers attended 5 sessions, each held in a different pool. For each session, water and ambient air CHCl3 concentrations were measured and CHCl3 levels in alveolar air samples (CHCl3ALV) collected from swimmers prior to entering the swimming pool premises and after 15, 35, and 60 min of swimming. Mean water concentrations varied from 18 µg/L to 80 µg/L, while those in air ranged from 78 µg/m3 to 329 µg/m3. Multiple linear regression analyses revealed that CHCl3ALV values in competition swimmers were strongly correlated to ambient air and water levels, and to a lesser degree to the intensity of training. Only ambient air concentration was positively correlated to CHCl3ALV in the leisure group. Concentrations of CHCl3 metabo lites bound to hepatic and renal macromolecules, estimated using a physiologically based pharmacokinetic (PBPK) model, were 1.6 and 1.9 times higher for the competition swimmers than for the leisure swimmers, respectively. The highest hepatic concentration predicted in competition swimmers, 0.22 µg CHCl equivalents/kg of tissue, was at least 10,000 times lower than the smallest no observed3effect level for liver tumors in animals. Data indicate that the safety margin is therefore very large, for competitive swimmers as well as for leisure swimmers.