In order to determine the appropriate treatment modality for roentgenographically occult bronchogenic squamous cell carcinoma (ROSCC), it is essential to evaluate the depth of invasion, because ROSCC invading beyond the cartilaginous layer cannot be effectively treated by photodynamic therapy (PDT) due to spread of disease. Transtracheal endoscopic ultrasonography (TUS) was useful for predicting the depth of invasion in some ROSCCs. In order to assess the actual significance of TUS as a diagnostic tool for predicting the depth of carcinoma invasion, we have conducted a prospective trial with 22 lesions of ROSCCs. We ultrasonographically classified the degree of the depth of invasion into two groups; A: "invasion does not reach cartilaginous layer" and B: "invasion involves cartilaginous layer". Then the patients were treated by irradiation, PDT, or surgical resection. Pathological findings were also classified into A or B. In order to calculate the sensitivity for evaluating the depth of invasion by TUS, the cases without any tumor and/or malignant cells after PDT were regarded as pathological A. In the evaluation of the depth of carcinoma invasion staying inside the cartilaginous layer, the sensitivity and the positive predictive value were 85.7%, the specificity was 66.7%, and the accuracy was 80.0%. With TUS, preoperative evaluation of the depth of invasion would be more accurate, and the decision of treatment modality would be more appropriate, compared with the conventional bronchoscopic observation alone.