BACKGROUND Use of insecticide-treated nets (ITNs) has become a central focus for the Roll Back Malaria campaign, and many countries in Africa have now embarked on large-scale public health programmes aimed at making ITNs available to those at greatest risk. However, the effectiveness of these programmes has rarely been evaluated. METHOD We conducted a cross-sectional survey to assess the impact of an ITN social marketing programme on Plasmodium falciparum infection and anaemia among children in urban Malawi. RESULTS Knowledge of ITNs was high; however, only 42% of the children surveyed reported to have used an ITN the previous night. Nevertheless, 17% (295/1721) of children had a positive P. falciparum smear at enrolment. Use of ITNs was associated with 52% protective efficacy against Plasmodium parasitemia. More than two-thirds of children were anaemic, yet the mean haemoglobin concentration was significantly higher in children using ITNs than in those not using nets. ITN use was associated with wealth, as poorer households were 60% less likely to use treated nets. CONCLUSION Although ITN social marketing programmes have the potential of improving malaria control and prevention, additional efforts are required to reach those for whom even subsidized nets are still too expensive.