A case-control study of leukemia and diagnostic X-ray exposure was conducted by a multi-institution co-operative study group. The subjects were 134 patients with acute myelogenous leukemia, 57 with chronic myelogenous leukemia, 56 with acute lymphocytic leukemia and 50 with myelodysplasia syndrome, who were between 15 and 79 years old, and diagnosed at one of 27 hospitals between September 1993 and August 1995. The controls were 479 first-visit patients seen at eight of these 27 hospitals. History of diagnostic X-ray tests between 1982 and 1991 was determined by an anonymous self-administered questionnaire. The total relative dose of radiation exposure was calculated by summing the products of given weights and frequencies of each test. The relative risk was 0.83 (95% confidence interval (C.I.), 0.58-1.19) for relative dose of 10-30 (equivalent to 4-11 times of UGI series), 0.76 (0.48-1.20) for relative dose of 30 or more (more than 12 times of UGI series), when compared with relative dose of 0-10 (0-3 times of UGI series). Analysis according to type of leukemia revealed that only acute myelogenous leukemia had an estimated relative risk above unity (1.08, 95% C.I. 0.69-1.69, for relative dose 10-30). This study did not support the hypothesis that diagnostic X-ray tests increases leukemia risk.