Lymphocyte bioenergetics in cancer patients undergoing surgery is largely unexplored. The effect of surgery on total lymphocyte counts and adenosine triphosphate (ATP) production is reported. Lymphocyte ATP was assayed by the coupled enzymatic method using hexokinase and glucose 6-phosphate dehydrogenase and lymphocyte counts were calculated from total leukocyte counts and differential counts. Lymphocyte ATP assay and counts were carried out in 13 patients pre- and postoperatively, and lymphocyte counts were scored in six more patients similarly. In 69% of the patients lymphocyte ATP production was depressed postoperatively, and this condition appeared to be associated with cancer spread and immunosuppressive tumors. Postoperative increase in lymphocyte ATP was observed following hysterectomy, colostomy, and also after blood transfusion. The significance of the findings are discussed. Changes in postoperative lymphocyte counts correlated with lymphocyte ATP in 62% of the cases.