The role of natural killer cell activity in immunosurveillance following thermal injury remains unknown. We utilize a chromium-51-release assay of K562 targets to monitor NK activity. NK activity of peripheral blood mononuclear cells of severely burned patients (BSA greater than 20%) was determined once a week until the convalescence period was completed. From the second week after thermal injury, impairment of NK activity was demonstrated, but the differences did not reach statistical significance. However, a correlation was found between decline of NK activity and incidences of septic complications. Surprisingly, no decline in NK activity was observed in the first week after thermal injury.