From 1978-1980, 20 patients with radiographically occult squamous cell carcinoma of the lung were admitted to Toronto General Hospital. Among them, five had in situ carcinoma, three had microinvasive carcinoma, and 12 early invasive. All cases were first diagnosed by cytologic examinations of sputum, localized by bronchoscopy, and had been verified histologically. It appears that a cytologic diagnosis of in situ squamous cell carcinoma of the lung could not be established on the basis of a single abnormal squamous cell or an occasional group of abnormal cells, but could be reached by putting together all the cytologic findings observed in the preparations. The criteria for the cytologic diagnosis are elucidated. Cytomorphologically there was no difference between microinvasive and advanced invasive carcinoma. However, in a patient with known localized in situ carcinoma and previous persistent cytologic findings of the same from that particular site, the finding of frankly malignant cells from the same site in the follow-up bronchoscopic examination was almost definitely an indication of microinvasive carcinoma, which usually showed evidence of both in situ and invasive carcinoma in the bronchial brushing specimens. None of the three microinvasive carcinoma cases had positive lymph nodes, but four of the 12 patients with early invasive carcinoma had metastases to the nodes at the time of surgery. Hence, the best time to effectively treat the invasive squamous cell carcinoma of the lung is at the microinvasive stage when the disease is considered curable.