It is a commonly held opinion that acid-fast staining of urine or gastric aspirates is not a reliable indicator of mycobacterial disease because of the presence of saprophytic mycobacteria. In order to determine the clinical usefulness of acid-fast staining of these body fluids, we reviewed our 10-yr experience with acid-fast stains. Forty-seven of 5,829 urine specimens (0.8%) and 39 of 309 gastric aspirates (12.6%) yielded mycobacteria on culture. Twenty-three urine specimens and 12 gastric aspirates had positive acid-fast stains. Of these, only one possible false positive acid-fast stain of urine (less than 1%) was found and none was found in gastric aspirates. Thus, acid-fast stains of urine and gastric aspirates are, when positive, reliable indicators of true mycobacterial disease.