This study was designed to verify our hypothesis that there are two different growth types in roentgenographically occult squamous cell carcinoma of the bronchus. Serial blocks prepared from the entire bronchial tree of 83 resected specimens of occult carcinoma were used for the evaluation of the relationship between the length of longitudinal extension and the maximal depth of transmural invasion. We prepared a length-depth diagram of 92 lesions, including multifocal carcinomas, which confirmed that there are at least two types: Most of these lesions are of the creeping type, which shows a marked superficial growth; the minority are of the penetrating type, which shows a marked downward growth. The diagram suggests that occult carcinoma has a propensity either for longitudinal growth along the bronchial lumen or for transmural growth into the bronchial wall at the time of occurrence. It is likely that the penetrating type grows rapidly and becomes advanced in a short time. Identification of longer lesions of the creeping type is occasionally problematical both at bronchoscopy and at surgical treatment. The stump is usually positive for carcinoma unless frozen sections or imprint specimens of the margin of resection are examined, because it is frequently difficult to identify the proximal end of extension by bronchoscopy.