The detection of the serum antibody in 122 schistosomiasis patients, using enzyme linked immunosorbent assay (ELISA), had been carried out from 1980-1989. The results showed that the potency of serum antibody was on an annual progressive decrease. Before treatment, the rate of ELISA reactive titre above 1:800 was 87.7%, while after treatment it was 21.2% in the first year and 1% in the second year and even lower in the following years. The antibody negative conversion rate with ELISA from the first to the fifth year after praziquantel treatment ranged from 53.2%-95.9%, whereas it was 97.0% in the tenth year. No significant difference was found between the ELISA positive rate in patients treated after 4 years and the false positive rate in normol persons (P > 0.05). The data demonstrate that it took four years after therapy for the noticeable reduction of the residual serum antibody level in patients, and that the ELISA positive reaction titre > or = 1:400 in patients 4 years after treatment might be used as complementary diagnostic index for those uncured patients.