A total of ten minute squamous cell carcinomas smaller than 1 mm were found in surgical lung specimens from 108 patients who had roentgenographically occult lung cancer. These minute lesions were detected by submitting, in all the 108 specimens, the whole bronchial tree to 2-mm-thick sequential transverse slicing which was then followed by microscopic examination of each slice on an H-E stained section. When a focus of minute carcinoma was found, the slice was further serially sectioned to study whether there were such carcinoma-related lesions as dysplasia or other atypical changes of epithelia, and when there were, the spatial relation of these with the carcinoma. It was demonstrated that all the minute carcinomas were closely associated with either dysplasia or what we call "basal cells with marked atypia", cells with markedly enlarged nuclei arranged in linear fashion on the basement membrane. The contiguity of these changes with minute carcinoma strongly suggested that they are lesions preceding overt carcinoma. Also, there were some minute foci of carcinoma, which, though not involving the entire epithelial thickness, proved to have already begun microinvasion.