Mycobacteria can be seen as "negative images" in cytology smears from patients with acquired immunodeficiency syndrome.

The cytologic diagnosis of mycobacterial infections has historically depended on the recognition of the characteristic granulomatous or purulent host response and the identification of acid-fast bacilli with special stains. However, immunocompromised patients with infections may not mount the expected response and the pathologist must have a high index of suspicion for the presence of organisms. We report the cytology findings from three patients with acquired immunodeficiency syndrome in which mycobacteria were seen on the routine modified Wright-stained (Diff-Quik) slides without special stains in the absence of granulomatous inflammation. The organisms appeared as negative images--unstained, rodshaped structures against the deep blue background of the stain. The findings were confirmed with Ziehl-Neelsen stain and culture. Implications for patient management are discussed.