To determine if there is any correlation between the knowledge of HIV/AIDS among in-school adolescents in Delta state of Nigeria and their sexual behaviour. A questionnaire based descriptive study of randomly selected secondary school students in Asaba. Delta state. A total of 437 students were recruited for the study. About 47% of respondents had good knowledge of HIV/AIDS. The electronic media were their main sources of information. Fifty-eight percent had been sexually initiated but only 10% were currently sexually active. About 73% of the males and 58% of the females reported condom use in their last sexual act. Seventy-three percent of the students that had poor knowledge of HIV/AIDS had ever had sex compared with 69.5% of students with good knowledge. Similarly 45.6% of students with poor knowledge used condom compared with 58.8% who had good knowledge. However these observed differences were not statistically significant. Knowledge of HIV/AIDS by in-school adolescents in Asaba has not significantly influenced their sexual behavior. Program planners should explore and integrate other factors that could impact positively on adolescent sexual behavior.