Secondary ciliary dyskinesia in upper respiratory tract. The system of mucociliary clearance has the important task to remove from the airways inhaled substances and locally formed secretions. Inborn disorders of the mucociliary transport are due to ciliary dysfunction (Primary Ciliary Dyskinesia) (PCD) or of increased viscosity of the bronchial secretions (Cystic Fibrosis). To differentiate PCD from the ultrastructural abnormalities found during or after injuries such as respiratory infections, the name of Secondary - or acquired - Ciliary Dyskinesia (SCD) was created. In controls, less than 4% of the cilia may show ultrastructural abnormalities. The most frequent of these are the compound cilia and the peripheral microtubular abnormalities. Compound cilia often appear after infection and therefore are thought to arise secondarily. Secondary ultrastructural abnormalities of cilia include also blebs of the axoneme membrane, ciliary disorientation, and absence of axoneme membrane. No increase in ultrastructural ciliary abnormalities has been found in a variety of respiratory disorders : smoking, asthma and allergic rhinitis, chronic rhinitis and sinusitis, chronic bronchitis, cystic fibrosis, and lung carcinoma. But severe modifications of the respiratory epithelium can be seen. Important for the secondary ciliary disorders is their local and reversible character. To distinct from ultrastructural images between primary and secondary ciliary dyskinesia is often uneasy because some of the findings in secondary ciliary dyskinesia obviously mimic those dedicated to primary ciliary dyskinesia.