This study examines the effects of class size upon the verbal interactions and the cognitive level of medical instruction. The major findings of this study indicate that: (a) the percentage of the teacher's verbal activities increases with the group size; (b) the shortest teacher-lectures are given in medium-size classes; (c) the longest student-answers and student initiations are observed in medium-size classes; (d) the maximal diversity of verbal interactions is found in medium groups, while the minimal is found in large groups; (e) the percentage of interactions at the lowest cognitive level (knowledge) increases with class size; (f) longer interactions are observed in large classes at the knowledge level, and in smaller classes at the analysis and evaluation levels; (g) the minimal cognitive diversity is found in large groups; (h) both activity and cognitive total scores decrease as the class size increases. The implications of these findings are discussed.