Sexual behaviour change remains a primary goal of HIV prevention efforts globally. It is a complex issue influenced by numerous unpredictable variables such as individual desires, social and cultural relationships, and environmental and economic dynamics. This study in northern Tanzania was conducted in June and July 2004 as part of a baseline survey carried out in 10 districts in eight regions of the country, Using a list of 65 villages in the northern zone, we randomly selected participants from four villages: one village each from three Area Development Programmes within World Vision Tanzania and one village from a high HIV transmission area (HTA). Five hundred and twenty-six respondents aged 15–24 years were selected from in-school and out-of-school groups. Of the total, 41.5% were sexually active. The age of sexual debut ranged from age 5–20 years for boys, and age 7–24 for girls. The mean age of sexual debut was 16 years old for males and 17 years old for females. The factors most strongly associated with sexual experience were: age greater than 16 years (p < 0.001); male gender (62.5% vs. 37.5%; OR = 2.4; 95% CI = 1.58–3.56; p < 0.001); living with both parents (47% vs. 27%; OR = 0.89; 95% CI = 0.84–0.94; p < 0.001); being out-of-school (75.8% vs. 24.2%; OR = 0.83; 95% CI = 0.73–0.94; p < 0.005); and, not using condoms during first sexual intercourse (OR = 1.46; 95% CI = 1.07–1.97; p < 0.02). Low self-efficacy in sexual relationships also predicted being sexually experienced (p < 0.001). High self-efficacy in intention to use condoms did not influence condom-use among sexually experienced youths (p < 0.001; OR = 1.70; 95% CI = 1.43–2.02). The perceived high influence of parents towards youths' intention to use condoms was a predictor of sexual experience (OR = 1.24; 95% CI = 1.05–1.45; p < 0.01). As a generalisation, the sexually experienced youths in the study population had experienced sex at a relatively early age and often had not used a condom at first sexual intercourse.