To evaluate the contribution of sputum cytology to the diagnosis and treatment of lung cancer, 918 cases of lung cancer were reviewed. Of 105 patients with positive sputum cytology, 93 had other invasive procedures to establish the cell type or stage or to attempt to cure the disease. In 12 patients the presence of positive sputum prevented further invasive evaluation. Thus, the real benefit of sputum cytology was reduced to 12 patients, or little over 1% of the total number of lung cancer patients. It is concluded that sputum cytology is not an effective diagnostic approach to suspected lung cancer. It rarely precluded invasive procedures because it failed to contribute to the staging of the disease. Therefore, it should be used in only those patients in whom the yield of positive sputum would direct therapy other than surgery.