BACKGROUND: During 1991 through 1993, sexually transmitted infections among conscripts in the Royal Thai Army in the upper-northern provinces were common: human immunodeficiency virus (HIV) prevalence at induction was 12%, HIV incidence was 2.4% per year, and incidence of sexually transmitted diseases was 17% per year. We evaluated a behavioral intervention to reduce incident sexually transmitted infections among conscripts inducted into the Thai Army in 1993. METHODS: We developed a preventive intervention that addressed consistent condom use, reducing alcohol consumption and brothel patronage, and improving sexual negotiation and condom skills. Companies were assigned to 1 of 3 groups matched on military mission: 450 men were in the intervention group, 681 were in barracks at the same base but did not receive the intervention (diffusion group), and 414 were in distant camps (controls). Baseline HIV serological testing and behavioral interviews were conducted during basic training in 1993. The intervention was applied for 15 months, and men were followed up at 6-month intervals (with repeated HIV serological testing, sexually transmitted disease assessments, and behavioral interviews) through May 1995. RESULTS: Incident sexually transmitted diseases were 7 times less frequent among men assigned to the intervention than the combined controls (relative risk, 0.15; 95% confidence interval, 0.04-0.55), after adjusting for baseline risk factors (P<.005). There was no diffusion of the intervention to adjacent barracks. The intervention decreased incident HIV by 50% in the intervention group. CONCLUSION: Intensive interventions in structured institutions can successfully reduce risk in settings confronting expanding heterosexual HIV epidemics.