The mortality experience between 1956 and 1985 of 8977 males employed by Atomic Energy of Canada Limited is reported. A total of 4260 men, 47% of the cohort, were exposed to low doses of external ionizing radiation at low dose rates, with a mean cumulative equivalent dose of 52.1 mSv. For cancers as a whole the excess relative risk, based on 227 deaths, was 0.36% per 10 mSv (90% confidence bounds -0.46, 2.45). This is quite comparable to the corresponding estimate based on the atomic bomb survivors study. There was a positive association between radiation dose and death from leukemia (excluding chronic lymphatic leukemia) P = 0.058. However, this was based on only four deaths and hence cannot sensibly be compared to estimates based on high-dose studies. The present results suggest that, for cancer as a whole, risk estimates based on high-dose studies are unlikely to underestimate risks substantially for low-dose and low-dose-rate exposures.