A retrospective cohort study was conducted for 12, 195 male radiological technologists who received the occupational exposure to low dose radiation over a long term. A total of 1, 097 deaths including 435 from cancer were ascertained by Koseki and death certificates from 1969 to 1993. Cancer mortality among the study population was basically compared with that of whole Japanese men. The significant low SMRs were obtained for all cancers, stomach and lung cancer partly due to Healthy Worker Effect, unlike the results of the early reports with some inappropriateness in the methods. Apparent high risks of lymphatic and hematopoietic cancers were observed, although none of site-specific cancers revealed the statistically significant increase. For these cancers, the SMRs among old sub-cohort were somewhat higher than those of young sub-cohort, whereas similar SMRs for solid cancer were obtained between the two subcohorts. The SMR for leukemia reached statistically significant level of 1.75 (95%CI:1.07-2.71) when using whole professional and technical workers as a standard population. The study results might suggest that the chronic exposure to low-dose radiation enhanced the risk of lymphatic and hematopoietic cancers. J Epidemiol, 1999 ; 9 : 61-72