Acute schistosomiasis mansoni was studied in 26 Puerto Rican patients whose clinical presentations differed widely in severity. Severity of illness was found to be positively correlated (r = 0.79) with the intensity of infection as measured by the concentration of eggs of Schistosoma mansoni in stool specimens. However, some patients had severe illness but relatively light infections. The disappearnace of symptoms and return toward normal of laboratory measures of disease activity were not associated with any diminution in the fecal egg count. Elevations of IgG, IgM, IgE, and of titers of anibody in serum indicated that the illness is associated with intense immune activity. The magnitude of the IgG and IgE responses was related to intensity of infection. The fact that incubation periods were shorter than the time needed for schistosomes to reach adulthood and lay eggs suggests that the syndrome can be initiated by parasite stages present before oviposition. No marked changes in complement (C3, C3p, C4, and CH50) and no signs of renal disease were noted in any of these patients.