The objectives of this study were to investigate the levels of knowledge attitudes toward and acceptability of emergency contraceptive pills among higher secondary school students, and also to look at factors that have an impact on the acceptability of the emergency contraceptive pills among these students. This study was an action research focusing on students both female and male in Bangkok in the academic year 1999. The sample consisted of 454 students. Multi-stage sampling was used. The data collection instrument was developed by the investigator, and was the primary source of data. Statistical analyses used were percentage, distribution, average and mean, paired t-test and logistic regression. The significant results of the study are: (1) Knowledge of emergency contraceptive pill: before the knowledge of the emergency contraceptive pill was given, the knowledge of this pill in both male and female students was low. After the intervention, they had a moderate level of the knowledge of the emergency contraceptive pill. The average score of knowledge after the intervention has risen with statistical significance. (2) Attitudes to the emergency contraceptive pill: before the intervention male students were neutral and after the intervention male students were positive to the emergency contraceptive pill, whereas the female students were neutral to this pill. (3) Acceptability of emergency contraceptive pill: before knowledge of emergency contraceptive pill was given, the majority of both male and female students had a perceived acceptability of the pill. After the intervention their perceived acceptability to the emergency contraceptive pill had increased. (4) Sexual experiences and experiences in the emergency contraceptive pill: about 20 percent of male students and 6 percent of female students had sexual experiences. Of these students more than half of the female students and a half of male students had at one time provided the emergency contraceptive pill for the female partner. (5) Factors influencing acceptability of emergency contraceptive pill: knowledge of and attitudes to emergency contraceptive pill, other factors did not seem to affect their acceptability to the emergency contraceptive pill. Recommendation : This study indicates that level of knowledge has an impact on the acceptability of the emergency pill. Hence, knowledge about the emergency contraceptive pill should be provided in schools. Therefore, schools which have an existing curriculum regarding contraception should add instruction related to the knowledge of emergency contraceptive pill into their curriculum.