OBJECTIVE: To obtain information about the social and cultural factors related to health behaviors influencing HIV/sexually transmitted disease (STD) transmission in study communities in China, India, Peru, Russia, and Zimbabwe so that the assessment and intervention of the National Institute for Mental Health (NIMH) Collaborative HIV/STD Prevention Trial could be adapted appropriately. METHODS: Field observations, focus groups, in-depth interviews with key informants, and an observation of community social dynamics were conducted as part of a rapid ethnographic assessment. RESULTS: All five sites reported a power dynamic tilted towards men, which rendered women particularly vulnerable to HIV and other STDs. Women's relative lack of power was exemplified by a double standard for extramarital sex, women's limited ability to negotiate sex or condom use, and sexual and physical violence against women. In all sites except Russia, extramarital sex is tolerated for men but proscribed for women. In Peru, power dynamics between men who have sex with men were tilted towards men who self-identified as heterosexual. Condom use (reported to be low across all sites) was often linked to having sex with only those perceived as high-risk partners. Regardless of site or study population, participants agreed on the following characteristics of an ideal community popular opinion leader (C-POL): respectable, credible, experienced (life and sexual), trustworthy, empathetic, well-spoken, and self-confident. CONCLUSION: The ethnographic studies provided critical information that enabled the study teams to adapt elements of the Trial in culturally appropriate ways in diverse international settings. © 2007 Lippincott Williams & Wilkins, Inc.