Cancer incidence (1950-1995) among 27,011 medical diagnostic x-ray workers was compared by means of O/E system with that of 25,782 other medical specialists employed between 1950 and 1980 to provide evidence of human malignant tumors produced by protracted and fractionated exposure to ionizing radiation and to assess resultant cancer risk. Significant cancer risk was seen among diagnostic x-ray workers (RR = 1.2, 95% CI: 1.1-1.3). Significantly elevated risks were found for leukemia and cancers of skin, female breast, lung, liver, bladder, and esophagus; the RRs were 2.2, 4.1, 1.3, 1.2, 1.2, 1.8, and 2.7, respectively. The patterns of risk associated with years since beginning x-ray work and with age and calendar year of initial employment suggest that the excesses of leukemia, skin cancer, and female breast cancer-and possibly thyroid cancer-were related to occupational exposure to x rays. Because of a lack of individual dosimetry for Chinese medical x-ray workers (CMXW) before 1985, the dose was reconstructed by physical and biological retrospective dosimetry methods. The cancer risk of CMXW was estimated based on the reconstructed dose. The average cumulative dose for the earlier cohort (employed before 1970) was 551 mGy, and for the later cohort (employed from 1970 to 1980) it was 82 mGy. The RRs of leukemia and solid cancer were significantly high for the earlier cohort: 2.4 for leukemia, 1.2 for solid cancer. But no significant increase of RR was evident for the later cohort. The RR of leukemia was 1.7 and 1.1 for solid cancer. This means a significant cancer risk can be induced by long term fractionated exposure to ionizing radiation when the cumulative dose reaches a certain level.