Thirty three patients, 24 male and 9 female, aged from 19 to 90 with an average of 58.8, were admitted to the medical intensive care unit (ICU) from 1988 December to 1989 December. Their host defenses were evaluated. Cell-mediated immunity (CMI) included delayed type skin test (MULTITEST CMI), total lymphocyte count and lymphocyte subpopulations (CD3, CD4 and CD8) were determined. Investigation of humoral immunity (HI) included use of serum levels of immunoglobulins (IgG, IgA and IgM) and complements (C3 and C4). Episodes of nosocomial infection were documented by patients' clinical and laboratory data, including positive culture. The CMI, especially for total lymphocyte count, T lymphocyte count and skin test, was impaired by underlying diabetes mellitus and such associated conditions as malnutrition, steroid administration and surgical procedures. The longer the admission period, the lower was the CMI including total lymphocyte count, CD3 percentage, CD4 percentage and skin test response. The HI was less impaired by underlying conditions and not influenced by admission duration. More infection episodes were found in patients with longer admission duration. In conclusion, the host defense was impaired in patients hospitalized longer in medical ICU, and the combination of compromised immunity and impaired mucocutaneous barriers made them more susceptible to infections.