This study investigated beliefs about HIV/AIDS to find out whether this variable could constitute an obstacle to change in attitudes to sex among undergraduate youths in South West Nigeria. A descriptive research design was adopted. A total of 1,420 undergraduate students in four different universities from four states were sampled. A user-constructed questionnaire entitled 'Sexual Behaviour and the Perception of HIV/AIDS' (SEBPHIV/AIDS). After validation it was administered to youths. The instrument has a construct validity coefficient of 0.76 and a reliability coefficient of 0.85. The statistical techniques used were descriptive statistics such as means, standard deviations, frequency counts and percentages, while Pearson product-moment and multiple regression analysis were used in analysing the data. The results showed a significant relationship between belief about HIV/AIDS and the sexual behaviour of youths. Belief was the best predictor of sexual behaviour. On the basis of the findings it was recommended that those involved in HIV/AIDS prevention and counsellors should focus on the sexual behaviour and belief of youths, especially undergraduate students, in order to bring about an effective and positive change in their attitude to sex. (Contains 3 tables.)