Severe periportal fibrosis is not an inevitable consequence of infection with Schistosoma mansoni. Genetic predisposition may be a deciding factor in the development of disease. To assess the contribution of genetic factors in the severity of hepatic fibrosis, the degree of familial aggregation was determined in a Kenyan population. Schistosomal fibrosis was identified with hepatic ultrasound and newly proposed World Health Organization criteria, which include both qualitative and quantitative observations. These 2 aspects of the criteria correlated well with one another. The peak prevalence of ultrasound proven fibrosis trailed 5-10 years behind peak prevalence of infection and declined sharply after age 50 years. This pattern was consistent with either resolution of severe fibrosis over 10-20 years or early death of those severely affected. Genetic predisposition appears to be a weak factor in the development of severe disease in this population, since no household or familial aggregation could be identified.