This study was conducted to determine the efficacy and long-term effects of an intervention programme based on a single dose of Ivermectin (6 mg) administered during a double-blind placebo study of 40 persons with Mansonella ozzardi infections in Blanchisseuse, Trinidad. After four years, ivermectin reduced microfilariae levels by 82.2%. Short- and long-term effectiveness of the drug is contingent upon the initial microfilariae levels of the patient. We conclude that a single dose of ivermectin reduces microfilariae densities and provides both short- and long-term reductions in M ozzardi microfilaraemia.