This study has been an attempt to investigate the impact of cooperative tasks on the lexical development of Iranian intermediate EFL learners. Sixty male and female students, who had scored homogeneously in a teacher-made proficiency test, participated in this study. Both the control and the experimental groups participated in a multiple-choice pre-test of vocabulary. After undergoing a course of eight sessions, through which the experimental group was taught through the use of cooperative tasks and the control group through the use of traditional techniques, both groups took part in a vocabulary post-test. The data analysis revealed a significant difference between the performances of the two groups, with the experimental group outperforming the other.