Fat grafting is used in combination with mammoplasty to improve filling of the upper pole of the breasts. Its effectiveness remains in question due to unpredictable results. Difficulty in isolating the grafted fat and differentiating it from host tissues may hinder assessment of graft integration. The plane between the pectoral muscles is free of fat and has already been described with respect to placement of breast implants and fat grafting in breast surgeries. This study sought to evaluate via magnetic resonance imaging (MRI) the integration and retention of retropectoral fat grafts in mammoplasty. Thirty patients with breast flaccidity who desired to undergo mammoplasty were selected. Fat collected from the abdomen was separated by sedimentation and transferred to the retropectoral region after undermining of the breast and resection of excess tissue. The patients underwent MRI preoperatively and at three and six months after surgery. Fat volumes were calculated by multiplying the values for the major vertical, horizontal, and anteroposterior axes by the constant 0.523. Twenty-five patients completed the study. The mean volume grafted was 116.4 ± 22.5 ml per breast. Six months after surgery, the mean fat graft volume in the retropectoral plane was 48.1 ± 25.71 ml, and the integration rate was 40.82% (range, 32.2-49.4%). The rate of complications related to fat grafting was 8%. In mammoplasty, retropectoral fat grafting showed good integration rates and is a safe and predictable approach that can contribute to improving the outcomes of aesthetic and reconstructive breast surgeries. This journal requires that authors assign a level of evidence to each article. For a full description of these Evidence-Based Medicine ratings, please refer to the Table of Contents or the online Instructions to Authors www.springer.com/00266 .