We conducted a study to examine the incidence of acute sinusitis following maxillary sinus fractures, as well as the impact of antibiotics in the postinjury period. Fifty patients who presented to our institution with a fracture of the maxillary sinus were prospectively randomized to receive either a nasal saline spray and a 3-day course of antibiotics (either amoxicillin/clavulanate or levofloxacin) or nasal saline alone; there were 25 patients in each group. After a minimum of 3 days, all patients were assessed for acute sinusitis. Follow-up data were available on 17 patients (a total of 21 fractured sinuses) in the antibiotic group (68%) and 14 patients (17 fractured sinuses) in the control group (56%). The groups were balanced in terms of demographics, location of fractures, mechanism of fracture, and time to follow-up. After 3 days, 95.23% of the fractured sinuses in the antibiotic group and 88.23% of the fractured sinuses in the control group exhibited signs or symptoms consistent with or suggestive of acute sinusitis. Very few sinuses in either group showed no evidence of sinusitis: only 1 in the antibiotic group (4.76%) and 2 in the control group (11.76%); this difference was not significant (p = 0.5768). We conclude that while the clinical rate of acute sinusitis after maxillary sinus fractures is high, a 3-day course of antibiotics is not effective in preventing its symptoms in the postinjury period.