Abstract: To further investigate the suitable dose-schedule of praziquantel for mass treatment of schistosomiasls, a comparative clinical study was carried out. 706 latent schistosomiasis cases were divided randomly into 3 groups: 1) 238 cases, each treated with a total dose of 50mg/kg in 1 day; 2) 244 cases, each with a total dose of 60mg/kg in 2 days; 3) 224 cases, each with a total dose of 72mg/kg in 3 days. The daily dose was divided into 3 portions given orally. A placebo containing yeast was given to 336 patients on the day or days when praziquantel was not administered to assess objectively the side-effects of the drug.All the patients except two tolerated praziquantel well and the course completed as scheduled. The main side-effects were dizziness, headache and fatigue. No significant differences were found among the 3 groups. However, higher percentage of side-effects were noticed in patients on the days receiving praziquantel than on the days receiving yeast. After treatment, except for 2 patients with premature beats, no remarkable changes were observed in liver or renal function tests, blood, urine, EEG or electromyogram on routine examination. Six months after praziquantel treatment, the stool examinations for miracidia in groups 1,2, and 3 were 100% (131/131), 96.9% (126/130) and 99.2% (127/128) negative respectively.The result suggests that the cure rates and side-effects were similar for the 3 groups. Praziquantel with a total dose of 50mg/kg in a day might thus be preferable for the mass treatment of schistosomiasis.