The diagnostic significance of a cytologic diagnosis of "severe dysplasia" on sputum samples was assessed. In a group of 46 patients with diagnoses of severe dysplasia, follow-up showed no malignancy of the lung in 25 patients (54%) and a malignant process in 21 patients (46%). These groups were compared to 52 patients with correct negative and 202 patients with correct positive sputum diagnoses. Of the patient characteristics investigated, age, previous sputum production, vital capacity and low forced expiratory volume were not significantly related to a sputum cytodiagnosis of severe dysplasia. In contrast, severe dyspnea showed a significantly higher frequency in patients with a sputum cytodiagnosis of severe dysplasia, but without an underlying malignant lung process. Follow-up disclosed a malignant tumor in 10 of 13 patients with disease; the presence of severely dysplastic cells in sputum specimens from such patients should be considered a warning signal for an underlying malignant lung process. Since severe dysplasia should be considered a premalignant epithelial lesion, patients with sputum cytodiagnoses of severe dysplasia should undergo bronchoscopy, with multiple bronchial brushings of all areas showing suspicious mucosal changes, together with segmental bronchial washings. In case a malignant process cannot be located, sputum examinations should be repeated at three-month intervals.