All Bellevue Hospital cases from a recent 27-month period whose fine needle aspiration (FNA) samples of cervical or supraclavicular masses showed acute and/or granulomatous inflammation were reviewed. The 30 patients included 8 with the acquired immune deficiency syndrome (AIDS), 3 with the AIDS-related complex (ARC), 2 with AIDS risk factors and 17 without known risk factors for AIDS. Of these, mycobacterial infections had been diagnosed in 22 patients: 18 by cultures positive for Mycobacterium tuberculosis and 4 by positive staining for acid-fast bacilli. In addition to the presence of neutrophils, two criteria for the diagnosis of mycobacterial infection were identified on the routinely stained FNA smears: caseous material and granulomas. Caseous material was the most sensitive and specific criterion. Granulomas were often present in patients with mycobacterial infection, but were also occasionally present in patients with other processes. The differences in cytologic specimens between AIDS and non-AIDS patients are discussed. The findings suggest that FNA is a safe and sensitive technique for the diagnosis of mycobacterial lymphadenitis in AIDS patients and that purulent aspirates from appropriate patient populations should prompt the use of special stains and cultures to rule out mycobacterial infection.