Abstinence is often promoted in HIV-prevention programmes for adolescents, but little is known about the factors that influence adolescents’ intentions to abstain from sex until marriage. This study was conducted in ten districts in the Kilimanjaro, Arusha, and Manyara regions of northern Tanzania, in July 2005. Out of 65 villages, four were randomly selected. In total, 953 primary school pupils, aged 10 to 14 years, participated in an interview and questionnaire: about 54% were girls and 41% were aged 12 to 13. Thirty-four percent of boys and 28.5% of girls said they had the intention to remain a virgin until marriage. Among the male respondents, having the intention to remain a virgin until marriage was associated with sharing a bedroom with a brother under age 18 years (odds ratio [OR] = 3.19, 95% confidence interval [CI] 1.48–6.87) and with saying that girls have the right to say ‘no’ to sex (OR = 8.51, 95% CI 2.93–24.72); among males, the intention was also negatively associated with saying they had the confidence to refuse sex with someone they had known for less than three months (OR = 0.12, 95% CI 0.04–0.32). Among the female respondents, having the intention to remain a virgin until marriage was associated with living with both parents (OR = 2.60, 95% CI 1.23–5.50) and saying that they had the confidence to refuse sex with someone who has authority or power (OR = 3.69, 95% CI 1.31–10.48). The findings highlight that young adolescents may benefit from community-based HIV-prevention programmes that include building lifeskills and increasing one’s confidence to abstain from sex or delay sexual debut until marriage.