Many diseases of the paranasal sinuses remain unidentified because of their asymptomatic course. The objective of the present study was to estimate the incidence of clinically manifested and asymptomatic fungal balls in the patients who had undergone the surgical intervention for the treatment of paranasal sinus diseases. The retrospective analysis of the case histories of 737 patients subjected to the surgical treatment of the inflammatory diseases of the paranasal sinuses was carried out. All the patients were examined prior to surgery with the use of spiral computed tomography (SCT) of the paranasal sinuses for determining their pathological contents, and the fragments of the mucous membrane that were investigated by means of microscopic, microbiological, and histopathological methods. The fungal balls were found in 57 patients who accounted for 7.7% of their total number. 49 patients of this group presented with the clinical symptoms of sinus disease; in 8 patients, the fungal balls were an accidental finding documented during skull SCT carried out for other indications.