During a 5-year period (1975–1980), 171 male patients aged eight to 58 years infected with Schistosoma mansoni have been treated with oxamniquine on an in-hospital basis. The patients comprised three clinical groups—uncomplicated S. mansoni infection, colonic polyposis due to S. mansoni, and hepatic decompensation due to S. mansoni—all with active infection. After treatment all patients were observed for 12 weeks then evaluated for cure. In the uncomplicated group 40 of 73 children (55%) and 39 of 45 adults (87%) were cured. In the polyps group 24 of 29 patients (83%) were cured and in the decompensated group all 24 patients were cured. The only side effect was a febrile reaction occurring in 65 patients (38%) 3–4 days after treatment was started. This 5 years' experience shows that oxamniquine results in a low cure rate in children but is effective in adults with uncomplicated S. mansoni. In patients with colonic polyposis oxamniquine is safe and well tolerated as initial therapy, particularly in the very ill, debilitated patient. Oxamniquine is clearly the drug of choice in patients with decompensated liver disease and active S. mansoni infection.