Peripheral lymphocytes, T and B cell counts, and skin test reactivity to five recall antigens were determined before operation and 6 to 18 months after operation with or without blood transfusion in 59 patients with inflammatory bowel disease. Significantly reduced preoperative peripheral lymphocyte and T cell counts reached normal levels 6 to 18 months after operation in patients who had not received perioperative blood transfusions. However, lymphocytes and T cells of transfused patients remained decreased, unchanged from preoperative levels, and significantly lower than those of control subjects (p less than 0.005) and levels of patients who had not been transfused (p less than 0.05). Before operation, transfused and untransfused patients were identical in age, Crohn's disease activity index, and peripheral lymphocyte and T and B cell counts. Equal numbers of patients were taking steroids and were anergic to skin tests with five recall antigens. Transfused patients weighed less preoperatively than untransfused patients, and these patients then gained a significant amount of weight. These results suggest that some of the immunologic abnormalities that accompany inflammatory bowel disease may be influenced by blood transfusions.