The Katayama syndrome, the early stage of schistosomal disease, occurs 3 to 6 weeks after infection. The main symptoms are fever, urticaria, oedema and general malaise. Eosinophilia is a constant finding. The syndrome developed in a group of Dutch tourists after a visit in early November 1975 to the Omo National Park, southwest Ethiopia. Eight out of 10 infected persons became clinically ill. The incubation period varied from 4 to 41 days with a mean of 26 days. Pyrexia occurred in 6 patients, usually associated with headache and muscle pains; only to one patient the fever lasted for more than two weeks. Fever followed by oedema was present in one patient. Two patients were afebrile, one suffered from urticaria, the other from general malaise. Two visitors remained asymptomatic, but the results of serological tests showed that they were also infected. The liver function was disturbed in one patient during the febrile period and further deteriorated during treatment with niridazole. S. mansoni eggs were detected in small numbers in two patients, 6 months and 19 months after infection. Obviously the tourists harboured few adult worms. They probably had been infected by few cercariae; the possibility that they were infected by cercariae of a S. mansoni strain not well adapted to man was considered.