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An increased prevalence of thyroid antibodies was seen in employees of a factory that formerly produced polychlorinated biphenyls (PCBs). In this study, the authors expand the evaluation of possible long-term PCB effects by comparing the prevalence of glutamic acid decarboxylase (anti-GAD) antibodies with the development of diabetes mellitus. The sera of 240 factory employees and 704 control subjects were analyzed. Anti-GAD antibody values exceeded 1.20 U/ml in all employees (40.4%), was 4 times higher (p < .001) than in all controls (10.5%), and were 5 times higher in employees aged 51-60 yr (53.2%) than in age-matched controls (10.5%) (p < .001). Although the prevalence of diabetes could not be determined from this retrospective study, this is the first report of a possible relationship between xenobiotics and the prevalence of anti-GAD antibodies, and it supports the concept of an immunomodulatory effect of PCBs. However, such antibodies may be present decades before the development of clinical diabetes, and not all anti-GAD antibody-positive individuals become diabetic. Presently, it is unknown whether there is an increased prevalence of diabetes among the former factory employees.