Six patients with hypoxic respiratory failure (arterial PO2/alveolar PO2 less than 0.50) resulting from active tuberculosis were evaluated to assess the impact of respiratory failure on the diagnosis of the underlying tuberculosis. All patients demonstrated anemia (hematocrit [mean +/- SEM], 0.29 +/- 0.01 [29.0% +/- 1.0%]) and hypoalbuminemia (serum albumin, 22 +/- 2 g/L [2.2 +/- 0.2 g/dL]) and noted an illness longer than one week. Findings on chest roentgenograms varied from a miliary pattern, misinterpreted as congestive heart failure, to cavitary and noncavitary alveolar infiltrates, misdiagnosed as bacterial pneumonia. Tuberculosis was not considered as a diagnostic possibility on admission in any patient. The mean time from admission until consideration of tuberculosis was 4.7 +/- 1.0 days and the time to diagnosis was 7.2 +/- 1.7 days. In contrast, tuberculosis was considered on admission in 12 patients presenting with undiagnosed active tuberculosis without respiratory failure. We conclude that respiratory failure delays the diagnosis of active tuberculosis by suggesting nontuberculous pneumonia.