Upper respiratory infections (URIs) are the most common organic condition presenting to the primary practitioner who cares for children. It recently has been estimated that approximately 5% to 10% of URIs in early childhood are complicated by acute sinusitis. Because children average six to eight colds per year, sinusitis is a very common problem in clinical practice. When considering a diagnosis of sinusitis in a child or an adult, the major problem is to distinguish simple URI or allergic inflammation from secondary bacterial infection of the paranasal sinuses. The diagnosis of URI or allergy may prompt consideration of symptomatic treatment, while patients who have sinusitis will benefit from specific antimicrobial therapy. Both URI and allergic inflammation are recognized risk factors for acute sinusitis, with URI being the more common. Anatomy and Physiology A brief review of the anatomy and physiology of the paranasal sinuses will help clarify certain clinical features of sinus infection. The Figure shows a coronal and two sagittal views demonstrating the relationship between the nose and the paranasal sinuses. The nose is divided in the midline by the nasal septum. From the lateral wall of the nose are projected three shelf-like structures designated according to their anatomic position as the inferior, middle and, seen best on the sagittal view, superior turbinates.