The kinetics of circulating anodic antigen (CAA) levels in urine were studied in Egyptian male patients infected with Schistosoma mansoni or with both S. mansoni and S. haematobium, before treatment, and at one, three and six weeks after chemotherapy. A quantitative enzyme-linked immunosorbent assay (ELISA) demonstrated CAA in 82% of the serum and 89% of the urine samples from these 28 patients. To evaluate the possibility of circadian variability in urine CAA levels, samples were examined in 15 patients at four intervals during a 24-hour period. No significant differences in CAA titers were observed. Seventeen patients were subsequently treated with praziquantel and followed for six weeks. CAA titers in serum and urine decreased significantly one week after therapy. Thereafter, the profile of CAA titer in urine continued to show a parallel but delayed decline compared to that in serum. While all serum CAA titers became negative three to six weeks after treatment, urine titers were negative in 47% at three weeks and 69% at six weeks. The remaining positive patients had low titers. A significant quantitative correlation in CAA titer was found between serum and urine before and after treatment. Seventeen Egyptian control subjects with no active schistosome infection were negative for CAA in both serum and urine. Our results confirm that the CAA urine assay could be used as a sensitive and non-invasive method to diagnose the disease, and indicate that the assay can be used to monitor efficacy of schistosome chemotherapy.