Introduction: Many investigations have recently been done regarding the effect of acute and chronic stress on immune system. Changes in peripheral blood leukocytes and their function can indicate health and disorder of immune system. The basic sciences exam is of importance for such students and is considered a stress or situation. The aim of this study was to investigate the effect of basic sciences exam on leukocyte subpopulations in peripheral blood of pharmacy students.Materials & Methods: A group of students, who were preparing for the basic sciences exam, were chosen as the case group. The control group included a number of students who had passed the exam the previous year. On the day of exam, blood samples were collected from the students. The percentage of leukocyte subpopulations including granulocytes, lymphocytes, monocytes and euosinophils were gained by hematological methods and T cell subpopulations including helper T (CD4 marker) and cytotoxic T (CD8 marker) cells, and also NK (CD56 marker) cells using monoclonal antibodies and flow cytometric technique. The data obtained from the two groups were analyzed statistically using t test.Findings: The results showed that stress caused significant reduction of CD4+T lymphocytes, monocytes, neutrophils, and CD4/CD8. However, CD8+T lymphocytes and eosinophil increased significantly. CD56+cells increased but the difference between the two groups was not significant. WBC count and lymphocytes did not show any significant differences in them.Discussion & Conclusion: It seems that stressor situations influence cells of immune system, especially those of T lymphocytes. Reduced CD4+T lymphocytes and increased CD8+T lymphocytes induced immune disorders in students, a fact that suggest more investigations.