We evaluated the impact of mass chemotherapy using praziquantel (40 mg/kg of body weight) on Schistosoma haematobium endemicity in a typical village in Giza, Upper Egypt. The entire village population of 988, determined by census, was included in the study. At baseline all villagers were screened for S. haematobium and offered treatment with praziquantel regardless of their infection status. Infection was determined by nucleopore filtration and egg counts were reported per 10 ml of urine. Information on compliance to treatment or refusal to participate was also obtained at baseline. One month after treatment, all those who were found infected at baseline were re-examined for infection and re-treated if found positive. Baseline was conducted in late fall just prior to the low winter transmission season. Approximately one year after baseline, the entire village was requested to participate in an annual follow-up urine examination and treatment. The results showed that the baseline prevalence was reduced by 83.6% from 23.1% to 3.8%. Geometric mean egg counts decreased four-fold from 12.4 to 3.1. There was a 60-fold reduction in the estimated population egg count from 10,006 to 167. There were only eight cases of reinfection and five incident cases. The typical age specific prevalence curve was flattened, showing that those in the childhood age groups benefited the greatest reductions in endemicity. Treatment noncompliance was 30%, for which half had justifiable reasons. The others simply refused treatment many of whom were nevertheless examined for infection. Mass chemotherapy was found to be a feasible and an effective method to reduce measures of S. haematobium endemicity in Egypt.