Objective: This study evaluated the knowledge and attitude of Tehran northwestern young population toward ecstasy use and its consequences. Method: In this descriptive, analytical-crossectional study, 400 young (247 males, 153 females) persons residing in northwestern districts of Tehran were assessed by a demographic and knowledge, attitude and behavior questionnaire. They were selected via the multi-stage random sampling. The data were analyzed through the descriptive statistic methods and X2. Findings: 41% of samples had good knowledge, 50% had moderate knowledge and 9% had low knowledge. 32% of samples had good attitude, 44% had moderate attitude and 24% had bad attitude. 26% of respondents have reported that they have used the ecstasy tablets. One important finding is that 31% of ecstasy users are also addicted to other kinds of narcotics. 44.9% of males and 35.3% of females were moderately aware of the dangerous outcomes of the ecstasy use. 25.9% of males and 20.3% of females regarded the ecstasy use a bad and dangerous act. In this study there was a significant relationship between attitude and gender (p = 0.035). Also there was a significant relationship between ecstasy use and gender(p < 0.001) and a notable relationship between ecstasy use and education(p = 0.008) Results: A considerable percent of respondents was not aware of the dangerous outcomes of the ecstasy and did not show the expected negative attitude toward this drug and a vast percentage of them had used the drug.