Objective: The aim of this study was to evaluate the efficacy of changes in the sum of the enhancement area lon- gest diameters (LD) in assessing no-surgical treatment response in bronchogenic carcinoma preliminarily. Methods: Twenty- two patients with bronchogenic carcinoma underwent two-phase contrast material-enhanced computed tomography prior to and after stopping no-surgical treatment more than one-month respectively. Two spiral CT scans were obtained at 25 and 90 sec respectively after nonionic contrast material was administrated via the antecubital vein at a rate of 3 mUsec by using an autoinjector. The sum of the tumor LD prior to treatment, after treatment and the sum of the post-treatment tumor enhance- ment area LD on the images obtained at 90 sec after injection of contrast medium were recorded. Enhancement patte was evaluated on the images obtained at 25 and 90 sec after injection of contrast medium. Response Evaluation Criteria In Solid Tumors (RECIST) guidelines were adopted to evaluate treatment response. The significance of the difference among groups was analyzed by means of ANOVA. Results: The sum of the tumor LD prior to treatment, that of after treatment and the sum of the post-treatment tumor enhancement area LD on the images obtained at 90 sec after injection of contrast medium were (4.48 + 1.19), (3.98 + 1.50), (3.35 + 1.11) cm respectively and there were statistically significant differences among them (F = 4.273, P = 0.018). The sum of the tumor LD prior to treatment was significantly higher than that of the post-treatment tumor enhancement area (P = 0.005). No statistically significant difference in the sum of the tumor LD was found between the pre-treatment and the post-treatment (P = 0.203). There was not statistically significant difference between the sum of the tumors LD prior to treatment and that of after treatment. According to changes in sum of the tumor LD, there were 4 of 22 (18.18%) partial responses (PRs), 14 of 22 (63.64%) stable diseases (SDs), 4 of 22 (18.18%) progressive diseases (PD) and response rate was 18.18%. According to changes in the sum of the post-treatment tumor enhancement area LD on the images obtained at 90 sec after injection of contrast medium, there were 5 of 22 (22.73%) PRs, 15 of 22 (88.18%) SDs, 2 of 22 (9.09%) PDs and response rate was 22.73%. Ten cases among 22 appeared homogeneous enhancement at 90 sec prior to treatment. At 25 sec, there were 6 cases with heterogeneous enhancement, 2 cases with homogeneous enhancement, 1 case with central enhancement, and 1 case with peripheral enhancement among the 10 cases. Six cases appeared homoge- neous enhancement and 16 cases heterogeneous enhancement at 90 sec after treatment. At 25 sec, there were 3 cases with heterogeneous enhancement, 2 cases with homogeneous enhancement, and 1 case with central enhancement among the six cases. Conclusion: Therapeutic effect may be underestimated with use of changes in sum of the tumor LD. The changes in sum of tumor enhancement area LD is suggested to be used in assessing no-surgical treatment response in bronchogenic carcinoma. The no-surgical treatment kill bronchogenic carcinoma cell and tumor after no-surgical treatment shows a gradual increase to the peak height after administration of contrast material.