This paper reports findings of a pilot survey of adolescent sexual and reproductive health (ASRH) knowledge and behaviour in Homabay County of western Kenya. The study was based on a cross-sectional survey of 523 male and female adolescents aged 10–19 years from 32 Community Health Units (CHUs). Bivariate analysis of gender differences and associations between ASRH knowledge and behaviour was followed with two-level logistic regression analysis of predictors of ASRH behaviour (sexual activity, unprotected sex, HIV testing), taking individual adolescents as level-1 and CHUs as level-2. The findings reveal important gender differences in ASRH knowledge and behaviour. While male adolescents reported higher sexual activity (ever had sex, unprotected last sex), female adolescents reported higher HIV testing. Despite having lower HIV/AIDS knowledge, female adolescents were more likely to translate their SRH knowledge into appropriate behaviour. Education emerged as an important predictor of ASRH behaviour. Out-of-school adolescents had significantly higher odds of having ever had sex (aOR=3.3) or unprotected last sex (aOR=3.2) than their in-school counterparts of the same age, gender and ASRH knowledge, while those with at least secondary education had lower odds of unprotected sex (aOR=0.52) and higher odds of HIV testing (aOR=5.49) than their counterparts of the same age, gender and SRH knowledge who had primary education or lower. However, being out of school was associated with higher HIV testing (aOR=2.3); and there was no evidence of significant differences between younger (aged 10–14) and older (aged 15–19) adolescents in SRH knowledge and behaviour. Besides individual-level predictors, there were significant community variations in ASRH knowledge and behaviour, with relatively more-deprived CHUs being associated with poorer indicators. The overall findings have important policy/programme implications. There is a need for a comprehensive approach that engages schools, health providers, peers, parents/adults and the wider community in developing age-appropriate ASRH interventions for both in-school and out-of-school adolescents in western Kenya.