Aspirates of 15 acutely and 13 chronically infected frontal sinuses were processed for aerobic and anaerobic bacteria. A total of 20 isolates (1.3 per specimen) were recovered from the 15 cases of acute frontal sinusitis, 16 aerobic and facultative isolates (1.1 per specimen) and 4 anaerobic isolates (0.3 per specimen). Aerobic and facultative organisms alone were recovered in 13 specimens (87%), and mixed aerobic and anaerobic bacteria were recovered in 2 (13%). The predominant aerobic and facultative organisms were Haemophilus influenzae (6), Streptococcus pneumoniae (5), and Moraxella catarrhalis (3). A total of 32 isolates were recovered from the 13 cases (2.5 per patient) of chronic frontal sinusitis, 12 aerobic and facultative isolates (0.9 per specimen) and 20 anaerobic isolates (1.5 per specimen). Aerobic and facultative organisms only were recovered in 3 instances (23%), anaerobes only in 7 instances (54%), and mixed aerobic and anaerobic bacteria in 3 instances (23%). The predominant aerobic bacteria were gram-negative bacilli (H influenzae, Klebsiella pneumoniae, and Pseudomonas aeruginosa). The predominant anaerobes included Prevotella species (8), Peptostreptococcus species (6), and Fusobacterium species (4). These findings illustrate the microbiologic features of acute and chronic frontal sinusitis.