School-based programmes to combat the spread of HIV have been demonstrated to be effective over the short-term when delivered on a small scale. The question addressed here is whether results obtained with small-scale delivery are replicable in large-scale roll-out. Primary School Action for Better Health (PSABH), a programme to train teachers to deliver HIV-prevention education in upper primary-school grades in Kenya demonstrated positive impact when tested in Nyanza Province. This article reports pre-, 10-month post- and 22-month post-training results as PSABH was delivered in five additional regions of the country. A total of 26 461 students from 110 primary schools in urban and rural, middle- and low-income settings participated in this repeated cross-sectional study. Students ranged in age from 11 to 16 years, were predominantly Christian (10% Muslim), and the majority were from five different ethnic groups. Results demonstrated positive gains in knowledge, self-efficacy related to changes in sexual behaviours and condom use, acceptance of HIV+ students, endorsement of HIV-testing and behaviours to post-pone sexual debut or decrease sexual activity. These results are as strong as or stronger than those demonstrated in the original impact evaluation conducted in Nyanza Province. They support the roll-out of the programme across Kenyan primary schools.