To determine the effect of perioperative blood transfusion on immunological parameters, T cells, T-cell subsets, and concanavalin A-induced suppression were measured in 25 patients with colorectal and breast cancer. During the operation, 15 patients received autologous blood and 10 patients had homologous transfusion. The immunological status was again determined after curative surgery. Before surgery, normal percentage of T lymphocytes, decreased ratios of helper/suppressor cells, and impaired con A-induced suppression were found. Following the operation, the helper and suppressor cell percentages reversed to normal, whereas the con A-induced suppression remained impaired. This change was significantly more pronounced in patients who received autologous blood transfusion than in the other group. Autotransfusion has an impact on immune parameters that might prove less detrimental to the clinical outcome in oncologic surgery than homologous transfusion.