A total of 105 lesions in 98 patients with roentgenographically occult bronchogenic squamous cell carcinoma were examined. The relationship of bronchoscopic findings to the depth of invasion into the bronchial wall and the length of longitudinal extension along the bronchus was documented. From viewpoints of the degree of difficulty of bronchoscopic detection and with reference to the height of the lesions, the bronchoscopic findings were classified into three categories: remarkable, minute, and hidden. Of the 105 lesions, 55 (52%) were remarkable, 27 (26%) were minute, and the remaining 23 (22%) were hidden. Of the 23 hidden lesions, 12 were within and 11 were beyond the range of endoscopic visibility. The maximal depth of bronchial invasion (mean +/- standard error) was 3.07 +/- 0.40 mm in the category designated remarkable and 1.62 +/- 0.47 mm in the category designated minute. The depth was 0.93 +/- 0.36 mm in the hidden lesions within the range of endoscopic visibility and 0.78 +/- 0.21 mm in the hidden lesions beyond the range of endoscopic visibility. The maximal length of longitudinal extension along the bronchus was 19.6 +/- 1.5 mm in the remarkable lesions, 9.9 +/- 1.4 mm in the minute lesions, 5.5 +/- 1.0 mm in the hidden lesions within the range of endoscopic visibility, and 8.6 +/- 2.1 mm in the hidden lesions beyond the range of endoscopic visibility. It is useful for predicting the depth of invasion to classify bronchoscopic findings into these three categories for the study of roentgenographically occult bronchogenic squamous cell carcinomas.