This paper reports studies perfomed on 28 sehistosome patients, with a known period of infection, who have been treated with hycanthone (in a single dose of 2.5 mg/kg, i.m.) in different periods after contagion. Coprological quantitative examination (Kato's method) has demonstrated the percentage of parasitological cure to be lower in newly-infected individuais. In fact, only 6 patients (40,0%) out of 15 treated from 2 to 8 months after infections-were shown to bs cured, whereas among 13, patients with one-year infection, no eggs could be recovered from the faces of 11 (84,7%) of the individuais, submitted to the treatment. However, even in newly-infected patients, hycanthone has induced a marked decrease in the number of S. mansoni eggs in the feces.