The majority of reports concerning nonpuerperal breast abscess (NPBA) identify aerobic and facultative bacterial isolates as the predominant flora in this disease. In the present study, fine-needle aspiration was performed in 29 women with NPBA; 12 (41%) of the patients had a history of chronic NPBA. The mean age of patients was 39.2 years. The aspirated material was cultured both anaerobically and aerobically. A total of 108 bacterial strains were recovered from 32 specimens; two specimens yielded no bacterial growth. A mean of 3.6 different bacteria were recovered from each culture-positive specimen. Anaerobic recovery outweighed aerobic-facultative recovery by a factor of 2:1. Significantly, 37 strains (five aerobes and 32 anaerobes) were harvested only from enriched broth subcultured for four to 14 days after initial culture processing. Coagulase-negative staphylococci (60% of total aerobes) and peptostreptococci (47% of total anaerobes) were the predominant bacterial isolates. These findings indicate that NPBA is due to a mixed flora with a major anaerobic component. Furthermore, the results suggest that routine cultures often overlook the involvement of anaerobes in these infections.