The accuracy of bronchial aspiration cytology in typing resectable (stage I-II) lung cancer has been investigated in 100 cases, comparing preoperative cytologic features with pulmonary tumor histology seen at surgery. The accuracy has been 100% for small-cell carcinoma (two cases), 98.8% for squamous-cell carcinoma (86 cases), and 91.6% for adenocarcinoma (12 cases). The overall accuracy rate has been 98%. No case of undifferentiated large-cell carcinoma has been identified. It is suggested that the high accuracy in cytologic typing of operable lung cancer is basically related to adequate preservation of differentiation features, thus allowing for correct identification of most non-small-cell carcinoma. Moreover, the absence in this study of any large-cell carcinoma, compared with its frequency in advanced stage series, would indicate that such a histotype reflects excessive dedifferentiation of an original squamous or glandular form.