A double blind trial was set up to study the pulmonary effects of specific treatment with oxamniquine of 40 patients with chronic schistosomiasis mansoni. Radiological alterations characterized by bronchopneumonitis were seen in 17.5% of the patients after treatment, but in none of the placebo group; non-migratory condensation occurred between 25 and 72 h after treatment, with a mean duration of 7 d. 86% of the patients showed spontaneous healing with no radiological sequelae after 30 d of follow-up. Slight pulmonary clinical manifestations without any functional repercussions were also seen. The alterations were probably related to the host-parasite interaction, and the lesions may have been caused by deposition of immune complexes in the lungs with local activation of complement.