The efficacy and safety of clarithromycin and amoxycillin in the treatment of acute maxillary sinusitis were compared in a single-blind, multicentre outpatient study. Fifty patients were randomly assigned to receive either clarithromycin 500 mg 12-hourly or amoxycillin 500 mg 8-hourly orally. Clinical signs and symptoms, sinus culture and blood and urine laboratory profiles were assessed prior to treatment, at four to six days during treatment, and within 48 h of the end of therapy (usually 9-11 days). Patients from whom beta-lactamase producing strains were isolated were excluded from the study. Both antibiotics achieved a clinical success rate of 91% within 48 h post-treatment; radiological resolution or improvement was observed in 91% of patients treated with clarithromycin and 89% of patients who received amoxycillin. Bacteriological cure was achieved in 88% and 91% of evaluable patients for clarithromycin and amoxycillin, respectively. Adverse events were reported for 16% of patients in the clarithromycin group compared to 26% in the amoxycillin group. Gastrointestinal disturbance was the most commonly occurring adverse event in both groups. The results of this study suggest that clarithromycin is as effective and well tolerated as amoxycillin in the treatment of acute maxillary sinusitis.