The objective of the study was to compare the cognitive function performance between a group of 30 cannabis users and a group of 30 non-users. The instruments used were a structured interview; Cognitive Screening (WISC - III and WAIS-III); WCST and Rey Complex Figure Test. The sample was 60 adolescents, aged from 14 to 17 from 5th grade on. The results showed that was statistically significant difference in the performance of the two groups in every neuropsychological test applied, and, in the Digit Symbol and in the WCST subtests, the users' group showed more impaired performance. The findings in this study it can be concluded that cannabis may affect the cognitive functions of adolescents.