The study of the relationships between Schistosoma mansoni and undernutrition deserves special attention bacause the foci of S. mansoni superpose undernutrition in underdeveloped areas of the world where the disease is endemic. Since the triad PARASITE-HOST-ENVIRONMENTconstitutes a system, malnutrition has been considered a factor which is able to modify the relationships between host and parasite aggravating the course of the disease. In the present paper different aspects related to the nutritionai status of the parasitized human host are reviewed (dietary pattern, frequency of clinical signs of deficiency diseases, biochemical changes in biood, anthropometric measurements), as well as the histopathological aspects of the skin and liver lesions in experimental/y infected albino mice submitted to different dietary levels of protein intake. In opposition to previous assertions, the data so far collected lead to the conclusion that undernutrition does not affect the course of Schistosomiasis considerably, although this parasitosis contributes, in some way, to aggravate a pre-existent malnutrition.