The impact of insecticide (permethrin)-treated bed nets (ITNs) on the health of adolescent schoolgirls was investigated during a community-based, randomized, controlled trial of ITNs in western Kenya. Two school-based cross-sectional surveys were conducted to determine the prevalence of malaria and anemia in 644 schoolgirls 12-18 years old in a rural area with intense perennial malaria transmission. In 12- and 13-year-old schoolgirls, ITNs were associated with a reduced prevalence of all cause anemia (hemoglobin level <12 g/dL, 16.9% versus 31.4%, adjusted odds ratio [OR] = 0.38, 95% confidence interval [CI] = 0.21, 0.69%) and a 0.34 g/dL (95% CI = 0.02, 0.66) increase in mean hemoglobin concentrations. No beneficial effect on all-cause anemia (adjusted OR = 0.79, 95% CI = 0.43, 1.45) or hemoglobin concentrations (difference in mean = 0.14 g/dL, 95% CI = -0.24, 0.53) was evident in older girls. In all age groups, no effect was found on malaria parasite prevalence or density, clinical malaria, all-cause morbidity, standard measures of nutritional status and growth, or the use of antimalarials and other medications. ITNs approximately halved the prevalence of mild anemia in young, school-attending, non-pregnant, adolescent girls, but had no impact in older girls or on other malaria-associated morbidity or nutritional status.