Lymphatic filariasis has been difficult to control until recently because of the lack of a suitable drug for treatment. Ivermectin has proven safe and effective at reducing levels of circulating microfilariae. However, the apparent need to administer the drug every 6 to 9 months to keep microfilaraemia levels sufficiently suppressed to reduce transmission has been a major drawback to using ivermectin in community-based intervention programmes. In a study conducted in Haiti, we have found that high doses of ivermectin suppress microfilaraemia levels for 2 years. Our findings suggest that a single dose of ivermectin can reduce transmission of lymphatic filariasis for extended periods of time, thus eliminating the need for costly biannual treatment.