The CT simulator is a real-time, CT-linked, 3-D treatment planning system which consists of a CT scanner, a multi-image display, a treatment planning device with real-time visual optimization, and a laser beam projector. The clinical benefit of this system was evaluated in breast cancer patients undergoing breast-conserving therapy. Between December 1987 and March 1993, 244 breasts in 243 patients underwent breast-conserving therapy at Kyoto University Hospital. Clinically, there were 159 stage I and 85 stage II tumors. Pathological staging showed 216 stage I, 24 stage II, and 4 stage III tumors. All except 10 tumors were invasive ductal carcinomas. Eleven tumors were margin-positive by histopathological examinations. As surgical treatment, quadrantectomy or wide excision with complete axillary dissection was performed. Radiation therapy consisted of 50 Gy delivered to the whole breast by opposed tangential fields over 5 weeks using 60Co gamma rays. For patients with a positive margin, boost electron irradiation was given to a total of 10 Gy. We used the CT simulator for individualized optimization of the tangential fields. Prompt and accurate determination of the tangential portals was possible by using the simulator to determine various parameters, including the beam angle and head rotation angle. During follow-up for 2-66 months (median: 20 months), unrelated death occurred in 2 patients and distant metastases developed in 5 (bone 4, lung 1). However, neither local recurrence nor symptomatic radiation pneumonitis has since occurred. The cumulative survival rate and the disease-free survival rate at five years were 95.2% and 91.8%, respectively. The CT simulator appears to be a useful tool for individualized optimization of tangential irradiation fields in breast-conserving therapy.