Determination of the efficacy of mass treatment of schistosomiasis is usually based on the rate of cure. However, schistosomes do not multiply in the human host, disease tends to cluster in the small proportion of individuals with heavy infections, and reinfection continually occurs in endemic areas. Thus drastic reduction in worm burdens can be a reasonable goal for mass treatment campaigns. The standard dose and regimen of metrifonate for the treatment of schistosomiasis haematobia is 7.5 mg/kg administered in three doses two weeks apart. This treatment results in a cure rate of approximately 50% and a reduction in egg output of 94.5%. In the present study, 72 infected children with egg counts before treatment that averaged from 0.1 to 2,334/10 ml of urine were treated with a single oral dose of 10 mg of metrifonate/kg: the cure rate was 22%, and the reduction in egg output was 96.5%. No side effects were recorded.