Sixty breast culture specimens were obtained by needle aspiration from 54 women and 2 men. A total of 221 microbial isolates were obtained from 52 culture-positive samples. Aerobes alone were recovered from 11 patients while 5 were culture-positive for anaerobes only. Thirty-six patients harbored mixed aerobic and anaerobic microbial flora, and the anaerobic gram-positive cocci were the predominant isolates recovered. The mean microbial recovery for patients with an acute abscess was 2.9 isolates, while in patients with chronic infections the mean microbial recovery was 5. The anaerobic populations outnumbered facultative isolates by two to one, and 34% of anaerobic isolates were recovered from subculture. These findings demonstrate that, contrary to previous reports, nonpuerperal breast infections involve a mixed infection that is primarily anaerobic.