The routinely recommended target volume for off-cord lung oblique fields in the treatment of postoperative bronchogenic carcinoma includes the entire mediastinum, as defined by coverage of the contralateral mainstem bronchus and subcarinal space. However, this may be difficult to accomplish with the field angles of 20° to 40°, recommended in the recently completed Intergroup Trial (Radiation Therapy Oncology Group 91-05). This project was undertaken to define the oblique angle necessary to encompass the entire mediastinum as determined by computerized tomography simulator verification. Axial computerized tomography simulation images of 25 patients with non–small-cell lung cancer were used in this study. Ten patients had prior lobectomy or pneumonectomy as part of their management. The contralateral mainstem bronchus, subcarinal space (SS), and the spinal cord were each contoured as separate volumes. The length of the contralateral mainstem bronchus was defined as extending from the carina to the bifurcation of the lobar bronchi. The subcarinal space was defined as a triangular space (in a coronal plane) with the carina at the apex, the mainstem bronchi superiorly, and a horizontal line 5 cm below the carina as the base of the triangle. The minimal angle to encompass the contralateral mainstem bronchus and subcarinal space, and to exclude the spinal cord was determined for each patient. The contoured volumes did not have additional margin added. The position of the carina was scored as “midline” if located in the midsagittal plane, or “off-midline” if deviated to either side from midline. Midline deviation was determined at the level of the carina to evaluate possible anatomical distortion relating to the tumor or prior surgery, and its effect on the minimal angle was assessed. The median minimal angle measured was 45° (range: 28–65°) for the entire group, and in 64% of those evaluated, this oblique angle was significantly greater than the 40° recommended in Radiation Therapy Oncology Group guidelines (p= 0.017). In patients without midline deviation (n = 17), the median minimal angle was 45° (range: 28–60°), and in patients with midline deviation (n = 8), it was determined to be 44° (range: 27–65°), with no statistical difference noted between the two groups (p= NS). Although midline deviation was present in 4 of 10 patients previously resected, the above relationship remained unchanged. Based on computerized tomography simulation verification, off-cord oblique field angles of 20° to 40° do not adequately cover the entire mediastinum in most patients. To adequately encompass the entire mediastinum as defined in the Intergroup Trial (Radiation Therapy Oncology Group 91-05) with off-cord oblique fields, treatment angles greater than 40° are necessary. Whether the potential increase in lung volume exposed to radiation from these larger angles results in a poorer therapeutic ratio requires further investigation.