Background and Objective: Chronic rhinosinusitis (CRS) is one of the most common chronic diseases. In the past decades, there has been an increase in fungal infections of sinuses and fungal rhinosinusitis (FRS) has been diagnosed more frequently. Knowing the fungal flora and its prevalence in CRS patients will allow a better understanding of this disease, permitting a correct diagnosis, treatment and formulating its prognosis. This study was performed to assess the fungal flora and its prevalence in CRS patients. Materials and Methods: In this prospective study fungal cultures were obtained from nasal and paranasal sinuses mucus of 100 patients suffering from CRS and 40 controls who did not show any evidence of CRS .In addition, in CRS patients, paranasal sinuses mucus and tissue were histologically investigated for evidence of eosinophilic granulocytes and fungal elements besides fungal culture. Results: Fungal cultures of nasal mucus were positive in 62.5% and 60% of cases and controls respectively. Positive fungal cultures of paranasal sinuses mucus were seen in 49% of cases and 5% of controls. Aspergillus was the most frequent, followed by Penicilium and Cladosporium in both nasal and paranasal sinuses mucus culture. Eosinophilic mucin was found in 90% of CRS patients. Conclusion: Our data showed that fungal infection is frequent in patients with chronic rhinosinusitis. Based on our results, Aspergillus was the most frequent isolated fungus in CRS patients.