Gynecomastia may occur as a normal physiologic development at certain ages or as a result of a variety of pathological conditions. An outbreak of gynecomastia was investigated at two processing centers of the Immigration and Naturalization Service (INS) between December 2, 1981, and May 14, 1982. At the Fort Allen Service Processing Center, Puerto Rico, gynecomastia was initially detected in 77 of 540 Haitian male entrants (14 percent) and in only 6 of 186 male employees of the center (3 percent) who were 18-50 years old; the difference in prevalence was statistically significant. At the Krome North Service Processing Center in Miami, Fla., gynecomastia was initially detected in 52 of 512 Haitian males 18-50 years old (10 percent). Two case-control studies did not demonstrate an association between gynecomastia and a number of factors that might have been related to an exogenous estrogen or to a substance with an estrogenic effect. Estrogen or estrogen-like substances were not found in food, water, or environmental samples. When the populations were rescreened several months later, 76 of the persons with gynecomastia detected in the first screening had had total or partial remission. Persons with remission had arrived earlier--a mean of 21.6 days for those at Fort Allen and 36.7 for those at Krome--than did those with newly detected gynecomastia and those with continuing cases. The difference in arrival dates was significant (P less than .005 for Fort Allen and P less than .001 for Krome). These results, in view of nutritional deprivation in Haiti, suggest that these cases may have been an outbreak of refeeding gynecomastia.