This study evaluated swimmers and visitors who were exposed to chloroform (CHCl3) at three indoor swimming pools in Modena, Italy. Chloroform was measured in plasma samples of 127 subjects present at the pools and in 40 nonexposed subjects. The analyses were performed by head-space gas chromatography. Chloroform was present in all samples collected from the 127 subjects who attended the pools (median = 7.5 nmol/l; range = 0.8-25.1 nmol/l). Agonistic swimmers who trained for competitions showed a significantly higher mean value of plasma CHCl3 than nonagonistic swimmers and visitors. Plasma CHCl3 levels were significantly correlated with (a) CHCl3 concentrations in water and in environmental air, (b) the number of swimmers in the pools, and (c) the time spent swimming. Covariance analysis showed that plasma CHCl3 levels also depended on the intensity of the sport activity (total explained variance = 67.4%).