This research examined the sexual behaviour and perception of the risk of HIV/AIDS and other sexually transmitted infections (STIs) among adolescents in Nigerian secondary schools. Quantitative data was collected from 450 senior secondary school 1 and 2 students who were selected from 5 of the 22 co-educational secondary schools in Ilesa using a multistage sampling technique. In addition qualitative data was collected by focus group discussion (FGD) with 8 groups (4 for each of the sexes) of ten randomly selected respondents from 4 other secondary schools. Sixty three per cent had had sexual intercourse. There was no statistically significant (p > 0.05) difference between the genders [females (52.3%) males (77.8%)], although males appeared to be more sexually active. The median age at first intercourse was 12 years with a range of 6 - 19 years. Many of the respondents had multiple sexual partners. There is poor perception of the risk of sexually transmitted infections (STIs) including that of human immune deficiency virus (HIV). It was concluded that there is a high risk of the spread of STIs, including HIV/AIDS in the study population. In view of the high prevalence of sexual intercourse and the reproductive health problems associated with STI, HIV/AIDS, it is recommended that a structured family life education (FLE) curriculum should be instituted for all schools starting at an early age.