We studied the fluctuations of schistosome circulating antigens in urine as compared with fecal egg counts in 60 Burundese individuals infected with Schistosoma mansoni. Levels of circulating anodic antigen (CAA) and circulating cathodic antigen (CCA) in the urine were determined by quantitative enzyme-linked immunosorbent assays. Fecal samples were simultaneously collected and examined with duplicate Kato-Katz slides. Significant correlations were consistently found between circulating antigen levels in urine and fecal egg counts. Although both antigen levels and egg output fluctuated, there was less fluctuation of CCA levels in urine than of fecal egg counts. All individuals had CCA in at least one urine sample and 82% were at least once positive for egg counts. Positive CAA levels were found in at least one urine sample in 75% of all individuals, but levels were low. Our results show that detection of CCA in urine is a sensitive, quantitative, and reliable method for noninvasive diagnosis and screening of S. mansoni infections, due to the relatively low fluctuations.