The present study was performed in order to pinpoint whether or not the oral penicillin treatment of the acute purulent maxillary sinusitis, which represents a common treatment modality in general practice, is in fact effective. The efficacy of penicillin V was tested in 32 patients under controlled conditions. In 24 out of 32 cases, which have been treated by administration of 4 X 1 tablet of Ospen 1500 per day, the acute signs and symptoms disappeared completely within a period of 7.4 +/- 2.02 days. The values obtained 4-5 hours after administration of 1 tablet of Ospen 1500 were significantly higher than the minimal inhibitory penicillin concentration for the bacteria tested. The plasma and sinus secret pencillin concentration time curves were found to be parallel. This implies similar kinetics of penicillin V in the plasma and the sinus secret. Since the present study shows that after oral administration penicillin is present in the nasal sinus secret in sufficient concentrations, oral penicillin therapy has been proven a necessful treatment of acute purulent maxillary sinusitis.