Surgery for cancer in man is followed by immunosuppression, and immunosuppressed cancer patients have advanced stage and poor prognosis. Yet surgery continues to offer the best chance for cure for patients with solid tumors. Since natural-killer cells appear to play a major role in control of tumor metastases in experimental animals, we studied peripheral blood T-cell subsets and natural-killer cytotoxicity of 75 patients with colorectal cancer preoperatively and between 1 and 3 months postoperatively. There was no relationship between preoperative lymphocyte, T-cell, helper-cell, suppressor-cell, or natural-killer cell number or natural-killer cytotoxicity and stage of disease. One to 3 months following surgery natural-killer cells had increased more than 50% from 218 to 325 cells/cmm (T = 2.07, P = 0.0459). Natural killer cytotoxicity increased proportionally. Lymphocytes, T cells, suppressor cells, and helper cells increased insignificantly. The results suggest that potentially curative excision of solid tumors in man is eventually followed by return of immune function reflected by increased numbers of natural-killer cells.