Background: The anatomy and tissue components of the infra-mammary fold (IMF) have been a matter of controversy among plastic surgeons and anatomists. This clinical study was done to detect variability in IMF anatomy (position and shape), to identify factors affecting its anatomical landmarks and also the influence of Lejour reduction mammoplasty on its position. Patients and methods: The first part of the study was done on 40 female volunteers examined clinically to define the shape and relations of the IMF to either the rib cage or the intercostal spaces. Data including age, marital status, lactation and breast volume were noted. The second part of the study included 20 cases operated on and followed up for one year after Lejour reduction mammoplasty to evaluate the effect of breast volume change on the position of the IMF. Results: The IMF is not a straight line or a simple curve; its medial end lies between the 5th and 7th rib while the lateral end lies between the 5th and 11th rib. The IMF intersects the breast meridian at the level of the 6th and 8th rib. The effect of breast reduction on the IMF is quite variable, it remained in the same level in 50% and elevated in 50%. Conclusion: IMF is a dynamic structure that undergoes a downward-oriented migration and thus proper positioning and fixation of the IMF on the thoracic wall in breast reduction is a crucial step to achieve the finest results