Since environmental chemicals have been shown to alter endocrine function, including sexual maturation, in wildlife and laboratory animals; this study examines if the ECs, polychlorinated biphenyls (PCBs) and dibenzofurans(PCDFs) may alter sexual maturation in humans. A cohort of adolescent children exposed in utero to very high levels of PCBs & PCDFs, were compared to a closely matched-control cohort of adolescents not exposed to high levels of PCBs and PCDFs in utero. On periodic physical exam the children's Tanner stage of sexual maturation was assessed. In the boys, size of testes and length of the penis were measured using standard techniques. There was no statistical difference in timing of the Tanner stages in either gender. However in the 55 pairs of boys between the ages of 11 and 15 years old, the penis length was decreased in the boys exposed in utero to PCBs and PCDFs. Testicular size was similar in both groups. Future studies will determine if these effects will persist when the boys are sexually mature, and the mechanism(s) of the observed changes.