Tuberculosis remains a significant clinical and public health problem in the United States. To determine if a significant proportion of hospitalized patients diagnosed as having pulmonary tuberculosis were not suspected of the disease following initial patient assessment, we retrospectively reviewed the microbiology records of a 515-bed community teaching hospital and identified all patients with culture-proven Mycobacterium tuberculosis infection hospitalized between January 1983 and December 1987. Pulmonary tuberculosis was not suspected in 13 (42%) of 31 patients with active disease. These patients were elderly (92% aged 65 years or older vs 8% aged less than 65 years), had a delay in respiratory isolation (6 vs 1 days) and diagnosis (8 vs 3 days), in addition to a longer hospitalization (16 vs 11 days) with increased mortality (46% vs 11%). Inadequate diagnosis may contribute to the persistence of morbidity and mortality from tuberculosis in this country.