This research compiles experimental studies from 1988 to 2010 that examined the influence of the cooperative learning method, as compared with that of traditional methods, on mathematics achievement and on attitudes towards mathematics. The related field was searched using the following key words in Turkish "matematik ve isbirlikli ögrenme, kubasik ögrenme, isbirlikçi ögrenme" and in English "cooperative learning and mathematics, meta-analysis." This study covered reports, articles published in refereed journals, and MA and Ph.D. theses. For the international literature review, advanced databases, such as ProQuest Digital Dissertations, EBSCO, and ERIC, were mined. A total of 26 studies (n = 36) were considered in the meta-analysis. The effect size for cooperative learning on academic achievement was found to be d++ = 0.59 (95% CI: 0.38 between 0.80) and the effect size for cooperative learning on attitudes towards mathematics was found to be d++ = 0.16. In terms of achievement, the effect size was found to be medium, positive, and significant, but for attitude, it was small, positive, and significant. As a result, cooperative learning was reported to be a more successful method than the traditional method with regard to both achievements and attitudes.