We have studied the impact of large-scale treatment with ivermectin on the transmission of loiasis in a forest village in south Cameroon where loiasis was highly endemic, with a prevalence of 30%. After one year of parasitological and entomological surveillance without treatment, all consenting residents aged > 5 years received ivermectin 200 micrograms/kg every 3 months. For ethical reasons, treatment was interrupted after 2 years, but parasitological and entomological surveillance continued for 18 months after the end of treatment. The prevalence of loiasis was reduced to < 10% and the mean microfilaraemia decreased by 90% in 2 years. The prevalence and average intensity of infection remained stable during the 18 months after treatment ended. Two vector species were identified, Chrysops dimidiata (representing about 90% of the fly population) and C. silacea. The infection rate (all stages) in Chrysops decreased by 75% and the infective rate (percentage of Chrysops harbouring third-stage larvae of Loa loa in the head) decreased by 85% in C. dimidiata and became zero in C. silacea. After the end of treatment, the infection and infective rates increased gradually. Large-scale treatment seemed an efficient method for the control of L. loa transmission provided high drug coverage was achieved. Nevertheless, because of the high risk of adverse effects when using the current microfilaricidal drugs, such a strategy remains unacceptable.