Mycobacterium (TB) is often a subtle disease. Unrecognized TB may occur in hospitalized patients and contribute to increased patient morbidity and even mortality. To determine the magnitude of this problem in the critical care unit, we retrospectively reviewed the records of all patients with cultures positive for TB who were admitted to the critical care units of the Rhode Island Hospital between 1981 and 1991. Fourteen such patients (mean age, 59; range, 31-84 years) wsere identified from a total of 100 cases of proven TB among all hospitalized patients during this 11-year period. Thirteen of 14 patients had risk factors known to predispose to TB, including alcoholism, malnutrition, and immunosuppression. No patient was known to have TB at the time of ICU admission. The median time from admission to definitive diagnosis or death was 25 days; TB was the primary or contributing cause of death in 9 of 14 patients. Reasons for the marked delay in diagnosis and subsequent high mortality include (1) a low yield of initial diagnostic tests for TB, (2) nonspecific radiographic studies, (3) willingness of critical care staff to attribute overwhelming illness to more common conditions seen in the ICU, and (4) empiric antibiotic or immunosuppressive therapy directed at nontuberculous processes. In all patients, TB was a strong diagnostic consideration but was dismissed when initial noninvasive and invasive studies were unrevealing. IN critically ill patients with unexplained fever and hypoxic respiratory failure, TB should be strongly considered despite negative diagnostic studies.