Feasibility of intraoperative cytodiagnosis of lung cancer. Of 354 thoracotomies, 114 cases involved intraoperative cytopathologic evaluation. The study included 86 men and 28 women, and 184 specimens were examined. Smears were taken from the lung lesion or lymph node and the chest wall or bronchial stump by imprint or needle aspiration. Intraoperative evaluations were compared with histologic findings. In 85 cases not diagnosed as malignant preoperatively, malignancy was confirmed in 97.6 percent of cases and histologic type in 71.8 percent. In the 28 cases diagnosed as malignant preoperatively, there was only one false-negative. The most important finding during thoracotomy is whether a lesion is malignant or not. The time required to obtain a pathologic diagnosis also is important. We are able to obtain the results of a cytologic diagnosis within 10 min. Cytologic diagnosis of malignancy during thoractomy may be a feasible and convenient method of diagnosis, especially where malignancy is suspected. 