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 less than 0.001). Marked mucosal thickening as determined radiologically (Water's view) was associated with abnormal aspirates whereas lesser mucosal thickening was not (P less than 0.001). In acute sinusitis, there was a strong correlation between high aspirate leukocyte counts (greater than 1000 per cubic millimeter) and infection as manifested by bacterial titers of greater than 10(5) per milliliter or the isolation of virus fungus (P greater than 0.001). Anterior-nasal-swab cultures correlated poorly with direct aspirate culters. Organisms frequently recovered from the sinus included Haemophilus influenzae, Strepto coccus pneumoniae, and anaerobic bacteria. Rhinovirus was recovered twice. Antibiotics were usedful in patients with acute sinusitis if the organism was sensitive in vitro (P less than 0.001).