It has been suggested that immunologic factors, rather than transmission factors, may play a particularly important role in the regulation of schistosome populations. Support for this view comes from several studies of the epidemiology of Schistosoma haematobium infection, although evidence is accumulating from both field and laboratory studies that immunity in schistosomiasis need not be a concomitant immunity. Correlations between immunologic parameters such as peripheral blood eosinophil counts and specific anti-worm immunoglobulin E (IgE) antibody levels, and apparent resistance to reinfection following chemotherapy have been reported and used as evidence in favor of the existence of an acquired immunity. Specific anti-worm IgG4 subclass blocking antibodies have also been described. The relationship between infection intensity and morbidity may not be as simple as has hitherto been assumed, especially since the methods used to generate data on the prevalence and intensity of infection have serious limitations that may only be overcome with improved or alternative sampling techniques.