Most exposures to low-level ionizing radiation, both diagnostic x-rays and nuclear radiation, occur in the range between 100 milirads and 10 radsthe "one rad range". In the past, the estimates of hazards in this range have been obtained by linear extrapolation from data on persons who were exposed to much higher dosages, generally in the centirad range used in radiotherapy of non-malignant disease. This article presents the first dosage response curve for the one rad range ever to be developed directly from data on men exposed to ordinary diagnostic radiation. The findings are based on approximately 220 men with nonlymphatic leukemia and more than 270 random-sample controls from the Tri-State Survey. The new findings