The internal mammary veins (IMV) are suitable vessels for thoracic wall reconstruction thanks to their relatively predictable anatomy and because they are less affected by atherosclerosis, injury or scarring of previous surgery and radiotherapy. Their position near to the lateral border of the sternum allows easier access for surgeon and the possibility of placing the most vascularized part of a flap for breast reconstruction in the medial thoracic region . As the complexity of reconstruction has increased, the use of the caudal portion of IMV has been reported as a convenient option for additional venous drainage. This procedure, requires retrograde blood flow that could be altered by the presence of efficient valves in the IMV. In this study, we evaluated 32 IMVs dissected from 16 fresh cadaver thoracic walls. Retrograde blood flow and the presence of valves were investigated. We observed an efficient flow in 18 IMVs, partial flow in 7 IMVs and no flow in 5 IMVs. In these last, single and/or multiple sacciform swelling and competent valves were macroscopically observed. Histomorphological analysis by Haematoxylin and Eosin and Masson-Goldner Trichrome staining confirmed their presence. In addition, some rudimental valves, not identified macroscopically, were found during histomorphological analysis. Taking together, these data highlight the possible presence of complete or rudimental valves in IMV. This aspect should be considered when retrograde flow of IMV as a single venous drainage was performed during surgical breast and thoracic reconstruction.