Since the early 1980s when the first case of HIV was diagnosed, AIDS remains a serious and threatening health crisis in the world. Sub-Saharan Africa is the region most affected by the HIV/AIDS pandemic having 67 % of the 33 million people infected with HIV globally in 2007. Young people aged 15-24 account for an estimated 45% of all new HIV infections worldwide. The study aimed to assess the knowledge and attitudes on HIV/AIDS and sexual behavior among 15-19 year olds in Kenya. The main findings indicate that a third of respondents were sexually experienced. Knowledge level was however found to be inadequate (below 60 %) indicating a poorly informed sample on knowledge about HIV transmission and prevention. The study showed poor attitudes towards the risk to HIV as only 40 % of both males and females supported the use of condoms even when the sexual partners know each other well. In addition, risky sexual behaviors were reported by sexually experienced respondents.