Estimates of the health risks in humans from low-level exposure to tritium are presented. The health risks considered are those for cancer, genetic effects, and develop-mental abnormalities from exposures in utero. Because direct risk information for these effects is not available from human exposures to tritium, the following approach was used. Excess risks for the effects given following low-level exposure to x rays or gamma rays were estimated from available human epidemiological data using appropriate dose-rate effectiveness factors. These human-risk estimates for low-level x rays or gamma rays were then multiplied by the appropriate best-estimate relative biological effectiveness for tritium, taking into account differences in effectiveness of comparison radiations. The resultant lifetime risk coefficients for low-level exposure to tritiated water are as follows. For cancer mortality, the most probable risk (50th percentile) is 81 × 10−6 mGy−1 and the 90% confidence interval is 38 to 185 × 10−6 mGy−1. For genetic effects in the first generation after exposure the risk is 7.9 × 10−6 mGy−1, with a 90% confidence interval of 3.8 to 16.3 × 10−6 mGy−1. For developmental effects from low-level tritiated water exposures in utero, the risk is uncertain but is estimated to be <400 × 10−6 mGy−1 The risks from exposure to organically bound tritiated most cules are estimated to range from values that are similar to those for tritiated water to about a factor of 2 higher.