With an increase in breast-sparing surgery and adjuvant radiotherapy, there has been a concomitant increase in the complexity of breast reconstruction. The effects of radiotherapy on the internal mammary artery were evaluated with respect to flap viability for conventional transverse rectus abdominis musculocutaneous breast reconstruction using the irradiated rectus muscle. Twenty-eight women who received postoperative irradiation for breast cancer were studied. All women had unilateral irradiation, and evaluation of the internal mammary arteries was performed at least 1 year after the completion of radiotherapy to allow for fibrosis and long-term vascular changes. Examination of the internal mammary artery (upper and lower chains) was performed using color Doppler sonography to assess vessel diameter, peak systolic velocity, and blood flow. The nonirradiated side was compared to the radiated internal mammary artery as an internal control. Based on the details of prior radiotherapy, the 28 women were designated into two groups. Group I included 14 women with radiation portals that specifically treated the internal mammary chain; the average dose to the internal mammary chain was 47.44 Gy. Group II comprised 14 patients that received tangential portals that did not directly target the internal mammary chain region; the average radiation dose was 48.21 Gy to the chest wall. No statistical difference was observed in group I between the irradiated and nonirradiated side for vessel diameter (p = .8631) or peak systolic velocity (p = .2646). However, an increase in blood flow on the irradiated side was significant (p = .0321).(ABSTRACT TRUNCATED AT 250 WORDS)