OBJECTIVE: To measure the impact on transmission of leishmaniasis of curtains impregnated with insecticide. DESIGN: Cluster randomised controlled trial: household interview survey, observational study of people's behaviour, entomological study with light trap captures of sandflies inside houses. SETTING: 14 urban sectors in Trujillo, Venezuela. PARTICIPANTS: 2913 inhabitants of 569 houses. INTERVENTION: Sectors were paired according to their 12 month cumulative incidence of cutaneous leishmaniasis, one sector in each pair was randomly allocated to receive polyester curtains impregnated with lambdacyhalothrin (intervention group) while the other sector received curtains without insecticide or no curtains (control groups). After 12 months a follow up household survey was conducted. MAIN OUTCOME MEASURES: Reduction in abundance of sandflies indoors and 12 month incidence of clinical cases of cutaneous leishmaniasis. RESULTS: Transmission of cutaneous leishmaniasis occurred mainly in the domestic setting, with the incidence over 12 months of 4%. The mean number of sandflies per trap per night was 16. After follow up the 12 month incidence of cutaneous leishmaniasis was 0% in the intervention group and 8% in the six pairs in the control group that received unimpregnated curtains (mean difference 8, 95% confidence interval 4.22 to 11.78; P=0.001). There were significantly fewer sandflies in the intervention group (2 v 15, mean difference 13 sandflies per trap; 9 to 17; P<0.001). CONCLUSION: Curtains impregnated with insecticide provide a high degree of protection against indoor transmission of cutaneous leishmaniasis.