Despite rapid progress in methods for analyzing radiation effects, much remains to be learned about the mechanisms and processes of radiation-induced immunological dysfunction. Among 17,899 sera obtained from atomic bomb survivors in Nagasaki, Japan, sera from 484 participants who complied with a reexamination for alkaline phosphatase (ALP) were tested for antimitochondrial antibody (AMA) by indirect immunofluorescence, and autoantibodies against 2-oxo-acid dehydrogenase complex (2-OADC) by immunoblotting to investigate the prevalence of primary biliary cirrhosis (PBC). Of these 484 sera, 28 (5.8%) were seropositive for AMA. The 484 participants were divided into three groups according to distance from the hypocenter: 72 who were exposed within 1999 m (closest group), 368 from 2000 to 5999 m (intermediate distant group), and 44 outside 6000 m (distant group). The positivity rates for AMA in these three groups were 6/72 (8.3%), 22/368 (6.0%), and 0/44 (0%), respectively (P =.08). Furthermore, high titers ( > 1:320) of AMA were observed in 3/6 (50%) AMA-positive sera from the closest group, in contrast to 4/22 (18%) from the intermediate distant group, although there was no significant correlation between AMA titer and distance from the hypocenter (P =.07). Of these 28 AMA-positive sera, 11 (39%) were from participants who had already been diagnosed with PBC, and 25 (89%) contained antibodies against at least one component of 2-OADC enzymes by immunoblotting. Therefore, the prevalence of PBC was estimated to be at least 615 cases per million (792 per million women). Our results suggest that the prevalence of PBC in atomic bomb survivors in Nagasaki is higher than that reported for the general population in Japan, and a further survey of the environmental factors, including radiation exposure, that predispose to PBC would be needed for understanding this disease of unknown etiology.