Examination of 6,880 sputum specimens from untreated patients disclosed that 3.1% were positive for mycobacteria by fluorescence microscopy, and 92.5% of these has positive cultures. There was a positive correlation between the number of organisms seen on smear and growth on culture. All specimens with positive smears and negative cultures contained rare or few acid-fast bacilli on the smear. Eighty-two percent of the specimens with positive cultures and negative smears yielded less than 25 colonies, whereas there were greater than 25 colonies from 93% of the specimens with positive smears and cultures. In a low prevalence, general hospital population, the primary acid-fast smear continued to be a reliable diagnostic tool.