Acute maxillary sinusitis is a disease of varied etiology. Over half of the cases are caused by Streptococcus pneumoniae and Hemophilus influenzae. Anaerobic bacteria account for another 10% of cases and these are usually of dental origin. The rest of the cases are caused by several other bacteria, each of which cause a small proportion. Rhinoviruses, influenza, and parainfluenza viruses also invade the sinuses and probably lead to secondary bacterial infection. Diagnosis of acute sinusitis on clinical grounds is difficult. Sinus transillumination and x-ray are the most valuable routine tests available. Ampicillin, amoxicillin, trimethoprim-sulfamethoxazole, and cefaclor have been shown to be effective treatment for most cases of acute sinusitis. Infection persists when there is inadequate or inappropriate treatment. The patient may become relatively asymptomatic in the face of persistent active infection. Follow-up clinical and x-ray examinations are indicated, when possible, to detect treatment failures. Although not a routine diagnostic procedure, sinus puncture and aspiration may be of value in the seriously ill patient or one who has not responded to treatment.