BACKGROUND The use of bariatric surgery to treat refractory obesity is increasingly common. The great weight loss that can result from these procedures has been shown to ameliorate certain deleterious effects of obesity. However, the effect of surgery on immune status is unclear. We investigated the relationship between surgical weight loss and peripheral blood lymphocyte percentages in women. METHODS Women (n=20, age range 25-59 years, body mass index [BMI] range 36.4-68.2 kg/m2) who had undergone either gastric banding (n=14) or gastric bypass (n=6) were enrolled in a prospective study to determine the percentages of their peripheral blood T cells (CD3+, CD4+, and CD8+), CD19+ B cells, and CD3-/CD16+CD56+ natural killer precursor cells before and 85+/-7 days (3 months) postoperatively using flow cytometry. The data are expressed as the percentage of total lymphocytes+/-the standard error of the mean. RESULTS A decrease in the BMI at 3 months postoperatively was 12% in the overall study population and 8% and 20% in the banding and bypass groups, respectively. No significant changes were found in the CD4+ or CD8+ T cells (P=.9 and P=.5, respectively), CD19+ B cells (P=.6), or natural killer precursor cells (P=.25) in the overall population or among the patients when stratified by surgical procedure (gastric banding or bypass). The change in CD3+ T cells approached significance (P=.06). A "same direction" (negative) correlation was found between the decrease in BMI and changes in the CD4+ T cell percentages between the pre- and postoperative levels in all the participants, and in the bypass and banding groups separately. However, it only reached statistical significance in the bypass group (r=-.96, P=.002). When studying the correlation between the decrease in BMI and the changes in CD3+ T cell percentages between the pre- and postoperative levels, a borderline significant negative correlation was found for all participants (r=-.44, P=.05) and in the bypass group (r=-.76, P=.08). The rate of change in the CD4+ and CD3+ T cells was greatest among those with the least weight loss and decreased with greater weight loss. CONCLUSIONS An inverse relationship exists between the change in certain T cells (CD4+ and CD3+) and the amount of weight lost after bariatric surgery, mainly gastric bypass surgery. The greater the decrease in BMI, the lower the change in these T cells.