Problem based learning (PBL) started to spread in health professions in Saudi Arabia at the beginning of this century. There are several challenges facing its implementation such as defects on interpersonal communications and self-directed learning. These challenges would affect students' performance in small group discussions and their achievement on exams. We believed that, introducing midweek sessions might improve students' performance. This intervention study included third year medical students (36 students). The students were randomly divided into two groups, A and B (18 students each). An extra small group discussion was conducted with group A in the middle of the week (midweek session) between the brain storming and debriefing sessions in haematopoietic and immunology courses. At the end of the midweek session, the students were asked to summarize and record their accomplishments. The scores marks plus or minus SD at the end of each course examinations were evaluated. Moreover, a questionnaire was designed to explore the students' opinion about the midweek session. The data were analyzed and the studied groups were compared using analysis of variance (ANOVA) and chi square tests. The means of the scores in continuous assessment and the end of the course exams were significantly higher in group A compared to group B in both immunology and haematopoietic courses. Moreover, a survey showed that the students have a positive feedback about the implementation of the midweek session. The midweek session has a good impact on the students' performance and achievement in problem-based learning. (Contains 2 figures and 3 tables.)