Similar to other countries, Iran is under influence of various and complex developmental and social factors which predispose young people to the risks associated with reproduction and sexual high risk behaviours. Therefore, any planning to develop appropriate strategies to meet their needs requires assessing the status of adolescent's reproductive health. The objective of this study was to explore knowledge, attitude and behaviour of Tehran adolescent boys (15-18 years old) with regard to reproductive health. This study was conducted among 1385 adolescent males aged 15-18 years in Tehran (Iran) in 2002 using the World Health Organisation questionnaire which was adapted according to Iranian culture. The method of randomized cluster sampling was applied. The results revealed a limited knowledge on reproductive physiology among Tehran adolescent boys (51.1%).A considerable percentage(12.6 %) of adolescents had absolutely no information about contraceptive methods. Among different contraceptive methods, condom and pills were known mostly by adolescent boys (72.2% & 58%, respectively). However the in-dept and correct information about condom was limited, so as 40% did not know that condom should be used once or 37% did not identified condom as an effective contraceptive method. Most adolescents (94.7%) had heard about sexual transmitted diseases previously but yet a great percentage of them held some misconceptions on STDs and HIV, so as, 16.6% were unaware about the lack of easy cure for AIDS at the present time and 23/1% from the existence of healthy appearance in HIV positive patients. Most (56.6 %) adolescents disagreed to any relationship between adolescents and their opposite sex. About one third (27.7%) of adolescent males reported an experience of sexual contact. Chi-Square test revealed that school drop out (P < 0.0001), education in night schools (P < 0.0001), lack of dependency on religion (P < 0.0001), absence of both parents in the household, father’s death (p=0.007), difficulties communication with mother (P=0.002), easy access to satellite (P=0.0017), smoking (P < 0.0001) and alcohol consumption (P < 0.0001) were among factors which were significantly related to the experience of previous sexual contact. According to the results of the present study, the high risk behaviours which might emerge due to the lack of appropriate knowledge among Iranian adolescents should be acknowledged more than before.