Total lymphocyte counts were monitored in 328 consecutive adult patients in the intensive care unit of a large community hospital. Lymphocytopenia was common with a severe reduction in circulating lymphocytes (less than 900/mm3) present in 37% of the patients and moderate reduction (900-1500/mm3) in 38% of the patients. Mortality was 30% and 13% in these two groups, respectively. Patients undergoing high-risk elective surgery frequently had moderate lymphocytopenia even when corticosteroids were not administered. Patients having two or more clinically evident infections with severe lymphocytopenia had a 59% mortality rate. Among patients who did not manifest any clinically detectable infection, mortality associated with those having severe lymphocyte reduction was significantly greater than those with normal lymphocyte counts or moderate depletion.