PURPOSE: The aim of this study is to evaluate the efficacy and mediation of a culturally appropriate, theory-based HIV/STI (sexually transmitted infection) risk-reduction intervention delivered in barbershops by barbers via iPads to African-American young men in reducing sexual risk behaviors. METHODS: In a cluster randomized controlled trial, 24 matched pairs of barbershops serving African-American men ages 18-24 in the 10 Philadelphia, PA zip codes with the highest HIV/AIDS prevalence were randomized to implement via iPads one of 2 interventions: "Shape Up! Barbers Building Better Brothers," an HIV/STI risk-reduction intervention based on the theory of planned behavior and formative research or an attention-matched violence-prevention control intervention. The primary outcome was self-reported consistent condom use 3, 6, and 12 months postintervention, controlling for baseline consistent condom use. RESULTS: Participants were 618 men, 319 in the HIV/STI intervention and 299 in the control intervention. Generalized estimating equation analysis indicated that the direct effect of the HIV/STI intervention in increasing consistent condom use postintervention was nonsignificant (odds ratio = 1.13, 95% confidence interval: 0.73-1.75), adjusting for clustering among participants in barbershops and baseline condom use. However, mediation analysis using the product-of-coefficients approach revealed indirect effects of the intervention. Consistent with the theory of planned behavior, the intervention increased behavioral beliefs and self-efficacy regarding using condoms, which raised condom use intention, which, in turn, boosted consistent condom use. DISCUSSION: Sexual risks among young African-American men can be reduced by barber-led theory-based, culturally appropriate HIV/STI risk-reduction interventions in barbershops in high HIV prevalence neighborhoods that increase behavioral beliefs and self-efficacy.