Background: The purpose of our study was to examine how physicians diagnose sinusitis in practice. We addressed three specific questions: (1) what clinical factors do physicians look for in evaluating and caring for patients with suspected sinusitis, (2) to what extent do physicians use transillumination and radiograph evaluations in diagnosing sinusitis, and (3) how does the diagnosis of sinusitis influence the decision to prescribe antibiotic therapy? Methods: We conducted a retrospective review using charts from 25 local family physicians who volunteered to participate in the study. After selecting a random sample of charts of adult patients treated for sinusitis and for upper respiratory tract infection (URI) by each physician, we reviewed the charts to determine the nature of the information collected to differentiate between sinusitis and URI. Results: Rhinorrhea, sinus tenderness, visualization of purulent secretions, and a history of sinusitis were significant predictors of the diagnosis of sinusitis. Antibiotics were prescribed for 98.4% of patients with sinusitis and 13.1% of patients with URI. Conclusions: This sample of physicians based the diagnosis of sinusitis on three prominent clinical findings, which were also significant factors in diagnosing sinusitis in previous studies. The history of sinusitis might influence patient and physician expectations for the diagnosis.