We have measured the levels of infection with Schistosoma haematobium in children resident in an endemic area of The Gambia before and 3 months after successful chemotherapy and following reinfection. An exposure index was calculated from data collected on water contact, cercarial densities and infected snail densities at water contact sites. Peripheral blood eosinophil levels were recorded and the ability of serum (heat inactivated) from the children to allow killing of schistosomula of S. haematobium was examined. Of 50 children with a post-treatment egg count of less than 1 ovum/10 ml urine, 26 were classified as reinfected, acquiring greater than 1 ovum/10 ml urine over the transmission season. Twenty-four were classified as not reinfected, acquiring less than 1 ovum/10 ml of urine over the same period. These two groups did not differ with respect to their estimated age, weight or pretreatment egg counts. Children who were reinfected had significantly higher levels of exposure and significantly lower peripheral blood eosinophil counts than children who were not reinfected. At all levels of exposure children with high eosinophil counts were less likely to be reinfected than those with lower counts. But antibody-dependent, complement-independent killing of schistosomula of S. haematobium by eosinophils was barely detectable and did not differ between reinfected and non reinfected groups. These observations suggest that subjects with elevated counts are less susceptible to reinfection but the mechanisms involved are not apparent.