A large-scale intervention field trial of the effect of Olyset long-lasting insecticide-treated bednets on transmission of cutaneous leishmaniasis was carried out in 2 cities in the Islamic Republic of Iran from October 2003 to July 2005. We enrolled 8620 individuals in 3000 households in 6 pairs of sectors in each city. Epidemiological and entomological surveys were carried out pre- and post-intervention. In both cities a statistically significant reduction was found in the incidence of new cases in intervention sectors who received bednets compared with control areas. Entomological surveys showed a reduction in numbers of female Phlebotomus sergenti captured indoors in intervention sectors.