BACKGROUND The proximal ends of internal mammary (IM) vessels are now the most common recipient vessels for breast reconstruction. On the other hand, bilateral deep inferior epigastric artery perforator (DIEP) flaps are often needed according to the territory and the volume required for reconstruction. The usefulness of retrograde IM vessels as second recipients has recently been reported, but there are very few quantitative studies on the hydrodynamics of the retrograde IM vessels. Because the flow is dependent on the pressure differential, the blood pressures of the antegrade IM artery (AIMA), antegrade IM vein (AIMV), retrograde IM artery (RIMA), retrograde IM vein (RIMV), and recirculated intraflap vein (FV) were investigated to solve this question and to confirm the reliability and usefulness of the retrograde IM vessels. METHODS Ten free flap breast reconstructions were included in this study. The IM vessels were exposed, and the pressures were measured. After recirculation, the FV pressures were measured when the flap was not ischemic or congestive. Systemic blood pressure was also recorded during the whole measurement period. RESULTS The AIMA and RIMA pressures were 70.4 ± 8.2 mmHg and 54.0 ± 8.6 mmHg (p = 0.000003), respectively, while the systemic pressure was 65.1 ± 10.0 mmHg. The AIMV pressure was always smaller than the RIMV pressure; the mean AIMV pressure was 5.3 ± 1.6 mmHg. In addition, the FV pressure was greater (p = 0.03) than the RIMV pressure (17.7 ± 9.9 mmHg), while the RIMV pressure was 8.7 ± 2.0 mmHg. CONCLUSIONS Both the RIMA and RIMV are useful and reliable as second recipients for bipedicled free flap transfers. This is a great benefit because it would provide two recipients in one surgical site and would be especially useful in thin patients or patients with previous abdominal scars requiring double pedicled DIEP flaps. LEVEL OF EVIDENCE Therapeutic Study, Level IV.