We evaluate the prognostic significance of preoperative natural killer (NK) cytotoxicity for K562 cells and its relationship to other prognostic factors in 102 patients with colorectal cancer who underwent curative resections between February 1984 and February 1985. The 18 patients who had recurrences within two years of surgery had significantly higher numbers of preoperative peripheral blood suppressor/cytotoxic and NK cells and significantly lower preoperative NK cytotoxicity than disease-free patients. Low preoperative NK cytotoxicity was predictive of recurrence independent of age, sex, hematocrit, procedure, blood loss, duration of surgery, Dukes' stage, specimen length, tumor size, tumor differentiation, and post-operative therapy. Low levels of in vitro NK-cell cytotoxicity may identify a subgroup of patients at high risk for recurrence.