In the present longitudinal study, a cohort (n = 98) of children and adults 5-30 years of age living in an area of highly seasonal and unstable malaria transmission were followed for malaria morbidity during several successive transmission seasons. Based on morbidity surveillance during 1993 and measurements of antibody titers to the Plasmodium falciparum ring-infected erythrocyte surface antigen (Pf155/RESA), the cohort was divided into three groups: those who had at least one episode of clinical malaria (Group 1, n = 31), those who did not suffer from clinical malaria but had (Group 2, n = 63) or had not (Group 3, n = 4) a significant increase in antibody titers against the Pf155/RESA antigen. This increase was defined as equal to or greater than a four-fold increase in antibody titer in samples from same individuals taken at the beginning and the end of the malaria transmission season. Such increases in specific antibody levels suggested that the donors had been exposed to a P. falciparum blood-stage infection. Measurements of antibody titers to a peptide derived from the glutamate-rich protein exoantigen gave data parallel to those for Pf155/RESA. A surprisingly high fraction of individuals in the study cohort (approximately 66%) showed evidence of infection without ensuing clinical disease (Group 2).