Nineteen cases of isolated squamous cell carcinoma in situ (CIS) of the bronchus were described clinicopathologically from among 149 male heavy smokers with roentgenographically occult lung cancer discovered mainly by mass screening performed from 1982 to 1991. All 19 patients had positive sputum cytology tests and negative chest x-ray films and underwent lobectomy (except one who had segmentectomy because of poor lung function). Prior to operation, localization was accomplished by one to eight bronchoscopies using repetitive brush cytology and biopsy. Five cases were bronchoscopically invisible. Polypoid protuberance was noted in three cases, micronodular swelling in three, thickening of spur in five, and mucosal granularity in three. Histology by serial block sectioning showed that there was no nodal involvement in any cases; the maximum length or diameter was 12 mm. Thirteen tumors were < or = 4 mm, four of which were confined to the spur where they occurred. Follow-up data showed a favorable prognosis. Segmentectomy or sleeve resection of bronchus without mediastinal lymph node dissection may be adequate for CIS.