Abstract: COP and fecal egg count were carried out on 324 proven cases in three endemic regions of schistosomiasis japonica of different status of snail infestation: 1. uncontrolled region, where Oncomelania is still present; 2. region of minimal transmission, where the snail has not been found for only one year; 3. controlled region, where snail has been eradicated for more than six years. The results revealed high sensitivity of COP for schistosomiasis japonica even in controlled region (97.8% positive among 46 patients) as well as in uncontrolled one (99.0% among 104) and in region of minimal transmission (96.2% among 174). But the intensity of the positive reactions was very different in the three regions, with 84.8% of the sera showing slightly reactive(+) in the controlled region, and 70% of the sera showing moderate or strong reactions (++ and +++). The intensity of infection of the patients was directly correlated with the percentage of COP egg reactivity in the three regions. It is interesting to note that the antibody level (represented by % COP egg reactivity) may decline slowly in the chronic patient even without treatment. Therefore in the controlled region, the COP egg reactivity of more than 1% might have diagnostic value for schistosomiasis.