Cell-mediated immunity was examined using 4 recall antigens in 102 critically ill surgical patients treated in an ICU. There were 4 (14%) deaths in 28 patients with positive reactions (R), compared to 33 (45%) deaths in 74 nonreactors (NR) (p< .01). Thirty-seven of the patients were admitted after trauma, and mortality in this group was not significantly different between R and NR patients. Repeat testing in 40 NR patients who remained in the ICU for more than 7 days showed that 30 remained NR (70% mortality) and 10 became R (20% mortality,p< .001).Peripheral blood lymphocytes from the patients showed a reduced response to concanavalin A, but a normal response to phytohemagglutinin and pokeweed mitogen. Mononuclear cells and granulocytes both showed a small increase in chemotactic response to casein, suggesting that the reduced response to skin testing may be partly due to defective T-cell function, rather than impaired phagocyte chemotaxis.