Introduction: The ultrastructural changes in paranasal sinus mucosa play an important role in the pathophysiology of chronic rhinosinusitis. The primary ciliary dyskinesia has been studied by many researchers. However, the secondary ciliary dyskinesia due to chronic sinusitis is still underappreciated. Objective: Describe the alterations of the mucosa ultrastructure of ethmoidal bulla in patients with chronic rhinosinusitis. Method: Twenty one patients with chronic rhinosinusitis were evaluated and submitted to histopathological examination and to an ultrastructural study with scanning electron microscope. Results: Light microscopy evidenced ciliated respiratory, cilindric ciliated, pseudostratified and squamous metaplasia epithelium. The metaplasia was difficult to detect in the light microscopy, but its presence was confirmed by electronic microscopy. In the lamina propria a discrete inflammatory infiltration was observed, constituted mainly by lymphocytes and plasma cells and more rarely by eosinophils and neutrophils. In the scanning electron, the changes had different intensities such as variable foci of normal cilia and foci of denuded epithelium with many cells showing microvilli. Another frequent finding was the areas with widened intercellular space between the epithelial cell. Conclusion: Chronic rhinosinusitis probably leads to a decomposition of cellular layer due to a loss of cohesion between its elements and the covering squamous metaplasia.