The rate of post mastectomy local-regional recurrence of breast cancer has remained in the range of 10-30% for decades. The traditional treatment, external beam radiation therapy, is successful in eradicating local disease in most cases, but re-recurrences are seen in about 50% of patients. Since 1982, 33 patients with such recurrences have undergone evaluation with computed tomography (CT) at our institution as part of their diagnostic work-up. In 22/33 (67%), CT revealed unsuspected disease, and in 10 of these patients the radiation treatment plan had to be altered. These results, similar to three other published series, strongly suggest that CT is a necessary part of the work-up of patients with post-mastectomy local-regional recurrences. The significance of these findings with respect to the cause of post mastectomy local-regional failures is further discussed.