Statistics show a rising incidence of HIV/AIDS infection in the Ejura-Sekyedumase District of the Ashanti Region in Ghana. In spite of this development, many people in the district have limited knowledge and serious misconceptions about the disease. Some deny its existence, while others attribute it to factors such as curses, witchcraft and mosquitoes. Discussions with parents, opinion leaders and young people also confirmed that premarital sex is on the ascendancy in the area, contributing to the increase in the pandemic. The research therefore sought to examine the effects of knowledge of HIV/AIDS and attitudes on the sexual behaviour of unmarried people aged 15-24 years in the study area. Both quantitative and qualitative data were employed. The quantitative data was obtained through questionnaire administered on a random sample of 450 unmarried people aged 15-24 years. The qualitative data was obtained through in-depth interviews and focus group discussions. The effects of knowledge and attitudes on sexual behaviour were and the testing of the hypotheses were done using the chi square tests, with p=0.05 as the level of significance. Other modes of analysis were frequency tables and bar graphs. There were four hypotheses for the study: (1) Knowledge of HIV/AIDS is significantly higher among people living in urban areas compared to those living in the rural areas. (2) Higher knowledge of HIV/AIDS results in less sexual activity. (3) Positive attitudes towards premarital sex results in less sexual activity, and (4) The higher the level of self-perceived risk of acquiring HIV/AIDS, the lower the level of sexual activity. The study used a modified form of the Health Belief Model as its conceptual framework. The results showed that 47.3% of the respondents had had sex. The mean and the median ages at first sexual intercourse were 17.5 years and 18 years respectively. The