To address global questions regarding the timing of HIV-prevention efforts targeting youth and the possible additional benefits of parental participation, researchers from the USA and The Bahamas conducted two sequential longitudinal, randomized trials of an evidence-based intervention spanning the adolescent years. The first trial involved 1360 grade-6 students and their parents with three years of follow-up and the second 2564 grade-10 students and their parents with two years of follow-up. Through grade-12, involvement in the combined child and parent-child HIV-risk reduction interventions resulted in increased consistent condom-use, abstinence/protected sex, condom-use skills and parent-child communication about sex. Receipt of the grade-6 HIV-prevention intervention conferred lasting benefits regarding condom-use skills and self-efficacy. Youth who had not received the grade-six intervention experienced significantly greater improvement over baseline as a result of the grade-10 intervention. The HIV-risk reduction intervention delivered in either or both grade-6 and grade-10 conferred sustained benefits; receipt of both interventions appears to confer additional benefits.