Allergic fungal sinusitis (AFS) is a noninvasive form of fungal rhinosinusitis with a prevalence of 6–9 % among all rhinosinusitis cases requiring surgery. The fungi causing AFRS have a great diversity and regional variation in the incidence of AFS has been reported worldwide. The aim of this study was to evaluate the prevalence of AFS among rhinosinusitis patients in the north east of Iran. In a prospective cross-sectional study 127 patients with sinonasal polyposis who were candidates for surgery were recruited. All cases were evaluated by sinonasal CT scan, Prick test for common regional allergens, total serum immunoglobulin E (IgE) level, and allergic mucin culture for fungi in suspected cases. Data analyses were done using SPSS version 13.0. Their mean age was 37.4 ± 11.6 years, the male-to-female ratio was 69/58. Eleven patients (9.45 %) met the AFS criteria. Nine cases had a positive culture for Aspergillus specimen and three for Alternaria. Prick test was positive for at least one allergen in 28 cases (22.05 %). The most common allergen was weed. The total IgE level was significantly higher in AFS patients. This study showed the prevalence of AFS among patients with nasal polyposis to be 9.45 %.