Twenty-four adults with clinical evidence of sinusitis were studied by 65 needle punctures of the maxillary antrum. Fourteen of 15 sinuses with normal transillumination and 19 of 26 that were dull had normal aspirates, whereas 24 of 24 that were opaque had abnormal aspirates (P < 0.001). Marked mucosal thickening as determined radiologically (Water's view) was associated with abnormal aspirates whereas lesser mucosal thickening was not(P < 0.001). In acute sinusitis, there was a strong correlation between high aspirate leukocyte counts (> 1000 per cubic millimeter) and infection as manifested by bacterial titers of > 10⁵ per milliliter or the isolation of a virus or fungus (P < 0.001). Anterior-nasal-swab cultures correlated poorly with direct aspirate cultures. Organisms frequently recovered from the sinus included Haemophilus influenzae, Streptococcus pneumoniae, and anaerobic bacteria. Rhinovirus was recovered twice. Antibiotics were useful in patients with acute sinusitis if the organism was sensitive in vitro (P < 0.001 ). (N Engl J Med 293:735–739,1975)