The aim of this study was to determine the most appropriate duration of treatment in acute maxillary sinusitis. The study was performed prospectively on 40 adult patients with acute maxillary sinusitis diagnosed by sinus puncture. Patients were randomized as to several treatment periods and treated by various antibiotics according to culture-sensitivity results. Patients in group I received treatment for 7 days; groups 2, 3, and 4 received 14, 21, and 28 days, respectively. The patients were followed up with nasal smear findings on certain intervals during the 56-day follow-up period. Statistically significant differences were found beginning from the 21st day between group 1 and the other groups. However, there were no statistical differences among groups 2, 3 and 4. These findings show that the most appropriate duration of treatment in acute maxillary sinusitis should be at least 14 days according to nasal smear results.