In May 1976 an investigation of a factory in Puerto Rico which formulates oral contraceptives revealed that during the previous 12 months five of the company's twenty-five employees (20%), and twelve of the company's thirty female employees (40%) had experienced symptoms associated with hyperestrogenism. The affected males had gynecomastia and three of them also reported a history of decreased libido or impotence. The affected females each had had at least one episode of intermenstrual bleeding during the preceding 12 months. There was an estimated relative risk of 4.3 for intermenstrual bleeding in nonclerical female employees compared with matched controls who did not work at the plant. Elevated levels of plasma ethinyl estradiol were twice as frequent in the two highest-risk job categories compared with the rest of the factory population, but the difference in prevalence of elevated levels was not statistically significant (P = 0.08). Wide variations in mestranol concentration were noted in the environmental dust samples. Prompt consideration should be given to establishing health standards for persons occupationally exposed to estrogens in view of the possible long-term sequelae of such exposure.