The authors have carried out a comparative study on the morbidity of schistosomiasis mansoni in four field areas of Brazil, two in the Southeast Region (Capitão Andrade, Itanhomi Municipality, and Padre Paraíso, both in Minas Gerais State) and two others in the Northeast Region (Riachuelo, Sergipe State and Alhandra, Paraíba State). The total sample studied in the four areas was 4,870, divided as follows: Capitão Andrade (area1) - 1,369; Riachuelo (area 2) - 850; Padre Paraíso (area 3) - 1,736; and Alhandra (area 4) - 915. In area 1, with a total population of 1,480 inhabitants, a study of the entire population was attempted. In areas 2, 3, and 4, due to the large number of inhabitants, a systematic sample by conglomerates was studied, of approximately 25 per cent of the population (family grouping of one in every four residences). The study consisted of an evaluation of the economic and sanitary conditions of the population, of contact with the local foci of transmission of schistosomiasis, of the frequency and intensity of infection by S. mansoni and of the relation between parasite load and the different clinical forms of the disease in different age groups. In parallel, a study was carried out of the intermediate hosts in each area and of the frequency of infection with S. mansoni cercariae. The prevalence of active S. mansoni infection was 60.8, 50.5, 63.1 and 46.6 per cent in areas 1, 2, 3 and, with a median egg elimination of 207, 77.6, 391 and 211 per gram, respectively. Progressive increases in frquency of infection of parasite load and of serious forms of the disease were observed from the first to the third decade in the areas of the Southeast Region and a later increase in the same parameters in the Northeast Region. Other correlations between clinical forms of the disease and age groups, sex, and ethnical origins of the patients, as well as frequency among the planorbids, were carried out.