OBJECTIVE This study was initiated to evaluate whether pretreatment diagnostic thoracic CT scan was useful for patients with loco-regional recurrent breast carcinoma, and to assess its impact on the design of radiotherapeutic treatment. METHODS Between March 1991 and January 1997, 44 patients underwent thoracic CT examination with contrast material before the consideration of radiotherapy for their isolated loco-regional recurrent breast carcinoma. The CT radiographs were prospectively reviewed for additional findings clinically undetected by prior physical examination and plain-chest radiograph. The changes made in treatment design and dosage of radiation as a result of CT findings were recorded for analysis. The correlation between prognostic indicators and the CT findings was also studied. RESULTS Twenty-two of 44 (50%) patients were found to have additional abnormalities detected only after thoracic CT examinations were performed. The strategy of radiation therapy was altered in 17 of 22 (77%) patients as a result. Patients with shorter disease-free interval (p = 0.08) and multiple sites of recurrence (p = 0.05) tended to have greater numbers of findings on CT scan previously unsuspected. Thus, CT scan is a valuable guide to treating loco-regional recurrent disease. CONCLUSIONS Pretreatment diagnostic thoracic CT scan offers essential information that can alter treatment planning and thus optimize treatment strategy for a large proportion of patients with clinically isolated loco-regional recurrent breast carcinoma. In this population of patients we recommend that thoracic CT examination be considered before the initiation of radiation therapy.