The occurrence of allergy was studied in 224 patients with verified acute maxillary sinusitis by means of an allergy questionnaire, skin testing, and nasal smears. Allergy was found in 56 patients (25%). In addition, allergy was considered probable in 14 patients (6.5%). The corresponding percentages in the control group were 16.5 and 3, respectively. The difference is statistically significant P less than 0.05). However, the frequency of allergy is lower in the present series than in those previously reported on chronic sinusitis. There were no differences between allergic and non-allergic patients in the number of prior acute sinusitis episodes or of previously performed sinus irrigations. Bacteriological and radiological findings did not differ significantly between the groups.