BACKGROUND: It has been suggested that reducing exposure to malaria by vector control might impair the development of naturally acquired immunity to malaria. It is also thought that an individual's ability to clear drug-resistant malarial parasites after treatment is enhanced by acquired immunity. METHODS: To investigate the hypothesis that insecticide-treated materials may affect the acquisition of immunity to malaria, we compared the ability of children living in villages in which insecticide-treated curtains (ITCs) had been used for 6-8 years to clear resistant parasites after treatment with chloroquine (CQ) with that of children living in unprotected villages. RESULTS: A total of 1035 children aged 6-59 months with falciparum malaria were treated with CQ; 409 were subsequently identified as carrying parasites with the pfcrt-76T allele. More children from ITC villages cleared parasites harboring this allele than did children from non-ITC villages (34.1% vs. 24.0%; adjusted odds ratio [OR], 1.80 [95% confidence interval {CI}, 1.15-2.80]; P=.01). The difference in the clearance of parasites with the pfcrt-76T allele was seen in children aged 6-35 months (32.3% vs. 19.3%; adjusted OR, 2.34 [95% CI, 1.18-4.66]; P=.02) but not in older children (37.3% vs. 37.0%; adjusted OR, 1.09 [95% CI, 0.56-2.10]; P=.97). Rates of adequate clinical response among children carrying parasites with the pfcrt-76T allele were similar in ITC and non-ITC villages (75.1% vs. 68.6%; adjusted OR, 1.21 [95% CI, 0.61-2.39]; P=.58). CONCLUSION: Our data suggest that the children who were protected from malaria by ITCs acquired functional immunity more rapidly than did the control children.