The present article describes the results of observations of 11 lesions in 8 cases of roentgenographically occult in situ or microinvasive squamous cell carcinoma with a size of 4 X 4 mm or less which were detected by detailed histologic investigations of 59 cases of occult bronchogenic carcinoma. The 59 cases were discovered mainly by mass screening for the detection of early lung cancers using chest x-ray combining sputum cytology for heavy smokers. The resected specimens were processed with the method of serial block sectioning. All the serial blocks of these minimal carcinomas were observed in detail to confirm the presence or absence of carcinoma and of morphological changes of the bronchial epithelium contiguous to carcinoma. Adjacent to carcinoma, there were normal bronchial epithelium in three lesions, squamous metaplasia with marked atypia in four, basal cell hyperplasia in two, and markedly atypical basal cells without hyperplasia in two. An inference on histogenesis of bronchogenic squamous cell carcinoma was drawn from the detailed observations as follows: (1) A carcinoma develops in the area of squamous metaplasia with marked atypia; (2) A carcinoma arises from markedly atypical basal cells with or without prior hyperplasia. Even in such small-sized carcinomas, there is a difference in type of invasion within the bronchial wall. One is the creeping type which shows a marked horizontal growth and the other is the penetrating type which shows a marked downward growth.