Whether low level radiation exposure before conception increases the risk of leukemia in offspring has been much debated. No study has specifically evaluated the effect of parental preconception diagnostic X-ray exposure in the development of leukemia among infants. Mothers of 302 infant leukemia cases (diagnosed at < or = 18 months of age) and 558 individually matched regional controls, and fathers of 250 cases and 361 controls, were independently interviewed to obtain information on X-ray exposures. Paternal preconception X-ray exposure was associated with an increased risk of infant leukemia, higher risks being linked to exposures closer to conception. X-ray related leukemia risk varied with exposure site and histopathological type, the highest risk being for acute lymphocytic leukemia related to two or more X-rays of the lower gastrointestinal (GI) tract and lower abdomen (odds ratio, 3.78; 95% confidence interval, 1.49-9.64). A positive association was observed between acute lymphocytic leukemia and number of paternal X-rays of the lower GI and lower abdomen (trend test, P < 0.01), upper GI (P = 0.04), and chest (P = 0.08). Exposures of head and neck and limbs were unrelated to risk. The risk of acute myelogenous leukemia was unrelated to paternal X-ray exposure, except for a marginally significant association (trend test, P = 0.07) for upper GI X-rays. No consistent association between maternal X-ray exposure and infant leukemia was observed. The results of this study suggest that paternal low level radiation exposure before conception is associated with an increased risk of infant leukemia, although the nature of this association needs to be further evaluated.