This paper investigates the relationships between knowledge of common sexually transmitted diseases and or HIV/AIDS and sexual behaviour change among adolescents in school and out of school in Nairobi City. Despite the existence of a well-structured health education system supported by informal health educators, it is apparent that health education knowledge acquired is not commensurate with desired changes in attitudes and behaviour. This study sampled 250 adolescents in Nairobi City aged between 14 to 24 years. Furthermore, focus group discussions were conducted to supplement views of individual respondents. The respondents socio-economic and cultural backgrounds represented well the major societal groups and classes. It was found that 93.2 percent of adolescents were knowledgeable of sexually transmitted diseases and over 90 per cent accurately identified symptoms of common sexually transmitted diseases and HIV/AIDS. However, it also emerged that peer pressure and peer networks have a very strong impact on attitude and behaviour of adolescent. The study recommends that more research is needed to understand how effects of peer networks could be made more effective in reducing adolescents pre-marital sex.