While male-to-male sexual behavior has been recognized as a primary risk factor for human immunodeficiency virus (HIV), research targeting men who have sex with men (MSM) in less-developed countries has been limited due to high levels of stigma and discrimination. In response, the Population Council's Horizons Program began implementing research activities in Africa and South America beginning in 2001, with the objectives of gathering information on MSM sexual risk behaviors, evaluating HIV-prevention programs, and informing HIV policy makers. The results of this nearly decade-long program are presented in this article as a summary of the Horizons MSM studies in Africa (Senegal and Kenya) and Latin America (Brazil and Paraguay), and include research methodologies, study findings, and interventions evaluated. We also discuss future directions and approaches for HIV research among MSM in developing countries.