In order to evaluate the clinical value of the saccharin test as a practical and simple measure of mucociliary clearance, nasal mucociliary clearance (NMCC) and ciliary ultrastructure were studied in 22 patients suspected of having primary ciliary dyskinesia (PCD) based on the saccharin test. Ten patients fulfilling the diagnostic criteria of PCD had a pathological response to the saccharin test (transport time greater than 60 minutes), and this was consistently associated with ultrastructural defects, specific for PCD. These results validate the suitability of the clinical use of the saccharin test as a screening procedure for NMCC. The false-negative results obtained in three cases of PCD, all with borderline values, cannot be ascribed to ineffectiveness of the test, but rather to the persistence of some motility by certain defective cilia, detectable by microphoto-oscillographic investigation of specimens obtained by nasal biopsy or brushing.