The bacteriological findings in 200 patients with acute maxillary sinusitis are reported. It is concluded that the sampling technique--by antral aspiration--is highly significant in the evaluation of the bacteriological background of sinusitis, whereas the anerobic transport of the sample seems to be of less importance. By aspiration the purulent secretion can be properly separated from the non-purulent secretion. Pneumococci, H. influenzae and anaerobic bacteria can be isolated in about 90% of the patients with true sinus empyema, while sterile conditions are rare.