Serum circulating anodic antigen (CAA) and circulating cathodic antigen (CCA) concentrations, as a possible direct measure of worm burden, were compared with fecal egg counts in a heavily Schistosoma mansoni-infected population from Zaire to allow differentiation between worm loads and worm fecundity in relation to age and intensity of infection. Of the 517 subjects, 95% excreted eggs and 97% demonstrated circulating antigens. Fecal egg counts showed an age-related pattern characteristic for an area in which schistosomiasis is endemic with intense transmission levels. Regression analysis showed that antigen concentrations were strongly associated with egg counts. For CAA, but not for CCA, this relation was found to be nonlinear, which would be consistent with density-dependent fecundity or crowding. The trend was uniform for all age groups, which for this particular population indicated a genuine reduction of worm loads rather than reduced worm fecundity with age of the host.