In 421 patients with a malignant lung process, from whom samples of sputum of satisfactory quality were received, patient characteristics relevant to the cytologic diagnosis of malignancy were investigated. In patients with primary lung cancer, the presence of blood in the sputum was highly significant from the point of view of its association with a correct positive cytologic diagnosis on sputum. The same relationship was noted in patients with metastatic lung cancer. In patients producing bloody sputum, the examination of at least three sputum samples gave a proportion of correct positive diagnoses of 0.88 in primary lung cancer patients and of 0.77 in patients with metastatic lung disease. Furthermore, a high sensitivity of the sputum cytology diagnosis of malignancy was found in primary lung cancer patients with low forced expiratory volume values (less than 50% of the vital capacity), with large tumors (greater than 24 mm in diameter) and with squamous-cell cancers. A central location of the tumor correlated with significantly better cytodiagnostic results in patients with both primary and metastatic cancers.