Elevated chronic psychological stress is associated with weakened immune response to vaccines. B-lymphocyte development may provide a pathway by which psychological stress can weaken immune response to vaccines. The current study examined the effect of chronic psychological stress on B- and T-lymphocytes among doctoral students, after their qualifying exams, and matched community controls. Blood was drawn from 10 doctoral students immediately after their 3-day qualifying exams and from 10 age- and gender-matched community controls. B- (CD19+) and T- (CD3+) lymphocyte percentages were enumerated with flow cytometry. Psychological stress was measured with the Perceived Stress Scale (PSS). The mean PSS score was higher for the graduate students compared with the control group (p < 0.01). Mean CD19+ lymphocyte percentage for the students (5.82 per cent) was dramatically lower than the control group (15.32 per cent, p < 0.0001). CD3+lymphocyte percentage did not differ between the two groups. There was a significant negative correlation between CD19+lymphocyte percentage and PSS score (r = -0.60, p = 0.003). Chronic psychological stress is associated with significant reductions in Blymphocytes among doctoral students undergoing qualifying exams and community participants. B-lymphocyte decrements are a potential mechanism linking psychological stress and reduced antibody response to vaccines.