Aims Sinusitis is the inflammation of the nasal cavity, and Paranasal sinuses occur by pathogen agents such as bacteria, viruses, fungi, or other allergens. This study was aimed to determine the prevalence of positive fungal cultures in patients with chronic sinusitis undergoing functional endoscopic sinus surgery.Materials & Methods This experimental study was carried out on 60 patients with chronic sinusitis in 2019. After anesthesia in the operating room, the ENT specialist evacuated the sinuses of patients with chronic sinusitis undergoing sinus endoscopic surgery. After sampling of evacuated sinus tissue of eligible patients and transfer to mycology laboratory, fungal elements were identified using direct smear and culture of the sample on subdextrose agar medium. The patient's profiles were recorded. Data were analyzed using SPSS 21 software by the descriptive statistic. Findings 36 of selected patients with chronic sinusitis were male (60%), and 24 were female (40%) with a mean age of 38.4±11.5 years. Two Candida parapsilosis (3.3%), one Aspergillus flavus (1.7%), and one Aspergillus niger (1.7%) were detected. Therefore, the prevalence of positive fungal cultures in patients with chronic sinusitis undergoing endoscopic sinus surgery was 6.7% in Yasuj, southwest of Iran, in 2019. ConclusionThe prevalence of fungal sinusitis is low in patients with chronic sinusitis in Yasuj due to the location of Yasuj in a mountainous region with a cold and dry climate.