Immunity to Plasmodium falciparum develops slowly in areas of endemicity, and this is often ascribed to poorly immunogenic or highly variant parasite antigens. However, among populations newly exposed to malaria, adults acquire immunity more rapidly than children. We examined the relationship between pubertal development and resistance to P. falciparum. During two transmission seasons in western Kenya, we treated the same cohort of young males to eradicate P. falciparum and then obtained blood smears each week for 4 months. We determined pubertal development by Tanner staging and by levels of dehydroepiandrosterone sulfate (DHEAS) and testosterone in plasma. In multivariate and age-stratified analyses, we examined the effect of pubertal development on resistance to malaria. In both seasons (n = 248 and 144 volunteers, respectively), older males were less susceptible than younger males. Age-related decreases in the frequency and density of parasitemia were greatest during puberty (15- to 20-year-olds). DHEAS and testosterone were significant independent predictors of resistance to P. falciparum parasitemia, even after accounting for the effect of age. Fifteen- to 20-year-old males with high DHEAS levels had a 72% lower mean parasite density (P<0.01) than individuals with low DHEAS levels. Similarly, 21- to 35-year-old males with high DHEAS levels had a 92% lower mean parasite density (P<0.001) and 48% lower frequency of parasitemia (P<0.05) than individuals with low DHEAS levels. These data suggest that the long period needed to attain full immunity could be explained as a consequence of host development rather than as the requirement to recognize variant or poorly immunogenic parasite antigens.