Ciliary function and mucociliary clearance are primary mechanisms of defense in the respiratory tract. We found that infections by several common respiratory viruses in children were associated with ciliary abnormalities that could be detected on ultrastructural examination of the nasal epithelium. Dysmorphic ciliary forms involving microtubular aberrations were observed most often in the early stages of illness in focal sites of the nasal mucosa. Normal epithelial organization and ciliary ultrastructure appeared to be reestablished during the convalescent period, from 2 to 10 weeks after infection. These observations suggest that interference with ciliogenic mechanisms leading to transient, compromised mucociliary clearance may represent a fundamental pathophysiologic disturbance in some respiratory viral infections.