Of 433 schistosomiasis mansoni patients in St. Lucia who were treated with hycanthone (3 mg/kg of body weight), 190 were seen 2 years after treatment and 143 of these had attended all follow-up examinations at 6 weeks, 6 months, 1 year and 2 years. Viable eggs were not detected in 86% at 1 year nor in 76% at 2 years. The reinfection rate, as judged by a significant increase in egg excretion, was 15% and was related to the geographic area to which the patient returned. Extremely high total reduction in egg excretion (98%) was achieved through 1 year, and even with reinfections this fell only to 87% at 2 years. Liver and spleen enlargement was related to intensity of infection and responded to treatment in 92% and 83% of instances, respectively. Among patients with hepatosplenomegaly, those 15 years or older showed less clinical response than younger patients but were too few for statistical comparison.