The study aimed to examine exposure to sexual content in mass media on TV, video/VCD, sexual websites and chat rooms on the internet and its effects on sexual attitude and behaviour. The sample was students aged 15-18 years studying in high-schools in Bangkok (a total of 390 students), selected by using multi–stage random sampling. Data was collected during November 2002. A self-administered questionnaire was used as an instrument for data collection. Statistical analysis used were percentages, mean, standard deviation, x^2 –test and Multiple Regression Analysis. The sample group comprised of 52 % females and 48 % males aged between 15-18 years, average student income was 2,900 baht per month. The majority of their friends had had exposure to sexual matters from the media (71.8 %). A reason for those exposing themselves to sexual content in the mass media was a response to their sexual curiosity. This study found that half of the students who had ever visited a chat room had dated with a new member of the opposite sex, and students who had had sex with a male or female they had known superficially met in a chat room. The variables significantly influencing the extent of sexual exposure are sex, having friends with sexual exposure, and perception of sexual matters. The sex variable can best describe the score variation of the extent of sexual exposure. Furthermore the result of analysis shows exposure to sexual content in the media was significantly influencing sexual attitude. Results from logistic Regression analysis demonstrated the influence of sexual behaviour. They were the extent of sexual exposure, sexual attitude and income.The odds of higher extent of sexual exposure being involved in sexual behaviour. This study explored some information for the programme planning of policy makers involved with youth S.T.D. and HIV prevention. The greatest source of information about sexual exposure as mentioned by students was friends, so using peer education is the most related method for sex education.