The microbiology of infections of the paranasal sinuses can be anticipated according to the patient's age, clinical presentation, and immunocompetence. In acute sinus disease, viral upper respiratory infections frequently precede bacterial superinfection by Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis. Staphylococci and respiratory anaerobes are common in chronic sinus infection, which may also be caused by exacerbations of infection with the bacterial species that cause acute disease. Enterobacteriaceae may be found in patients with nosocomial sinusitis who are predisposed to the development of sinusitis by prolonged nasogastric and nasotracheal intubation. Immunosuppressed patients have episodes of sinusitis caused by the usual agents associated with acute sinusitis in immunocompetent patients, and they may also become infected with a broad array of unusual agents, including mycobacterial species, fungi, and protozoa.