Cellular immune responses to the Plasmodium falciparum circumsporozoite (CS) protein were measured by proliferation and interferon-gamma production in a cohort of children aged 3 to 8 years, living in The Gambia. Anti-CS antibody titres, malariometric indices and sickle cell status were also determined. Malaria morbidity in the ensuing malaria transmission season was monitored by weekly health questionnaire, axillary temperature measurements and examination of blood films. Exposure to malaria was inferred from entomological data collected during the transmission season. Immunological and parasitological measurements were repeated at the end of the rainy season. Immunological findings were compared between children who experienced clinical malaria or asymptomatic infection and children who had no evidence of infection. No association was found between cellular immune responses to the CS protein at the beginning of the transmission season and subsequent susceptibility to infection except among children with high titres of antibody to (NANP)40. Seropositive children who did not become infected had a higher mean proliferative response to the Th3R epitope than seropositive children who did become infected. High titres of anti-(NANP)40 antibodies alone were not protective. Responses to the Th2R epitope were significantly higher at the end of the rainy season than at the beginning in children who experienced an asymptomatic infection. Responses to variant sequences of the 2 epitopes were highly correlated at an individual level but there was no correlation between proliferative and interferon responses to a particular epitope.