To study the association between hepatitis B and schistosomiasis, 1,234 Egyptian males, ages 18 to 24, were interviewed, examined, and tested for Schistosoma mansoni infection and HBsAg. Sera from 91 (7.4%) of the study subjects were positive for HBsAg, and S. mansoni was found in the stools of 26.3%. There was no correlation between S. mansoni infection, with or without hepatosplenomegaly, or a history of schistosomiasis, and HBsAg. An association was found between HBsAg positivity and a previous history of parenteral antischistosomal therapy (P less than 0.01). The results of the study indicate that parenteral therapy for schistosomiasis may be a risk factor for hepatitis B antigenemia. Further studies are indicated to determine the importance of parenteral therapy in the transmission of hepatitis B.