INTRODUCTION: Although body mass index (BMI) > 30 kg/m2 is often considered a relative or absolute contraindication for abdominally -based free flaps, there is very little evidence in the literature that BMI correlates with complication rate. The purpose of this study was to determine whether BMI or abdominal wall thickness correlated with postoperative complication rate. METHODS: A retrospective chart review was conducted on 151 patients who underwent abdominally based free flap reconstruction after mastectomy at our institution. Preoperative axial computed tomography scans were used to measure the distance from the skin surface to the abdominal wall at the umbilicus and at the lower abdomen (8 cm inferior to the umbilicus). The medical records of all 151 patients were then scanned for demographic data, medical history, BMI, surgical records, and post-operative complications. Mean BMI, mean abdominal wall thickness at the umbilicus, and mean lower abdominal wall thickness (LAWT) were then compared between patients who did and did not develop overall, abdominal donor site, and breast recipient site complications. The mean LAWT was also compared between patients who had and had not experienced different subtypes of abdominal and breast complications (unpaired t test). Patients were then stratified into four groups based on LAWT, and complication rates were then compared between all four groups (chi-square test). RESULTS: Average BMI did not differ significantly between patients who did or did not experience any complication, abdominal complications, or breast complications. Mean abdominal wall thickness at the umbilicus was significantly higher in patients with overall complications (P = 0.005) and abdominal complications (P = 0.003) but did not differ significantly between those who did and did not experience breast complications. Average LAWT was significantly higher in patients who experienced all three types of complications (P = 0.001, 0.0001, 0.02, respectively), and so this point was chosen as the major comparison point. Patients with abdominal complications requiring re-operation (P = 0.001), abdominal wound healing complications (P < 0.0001), and breast wound healing complications (0.004) had a higher average LAWT compared with those without these respective complications, but there was no significant difference between those who did and did not experience abdominal infectious complications, breast complications requiring reoperation, breast fat necrosis, or breast infectious complications. After stratification of complication rates based on LAWT groups, the rates of overall abdominal complications and abdominal wound healing complications significantly increased as LAWT increased (P = 0.01, 0.03, respectively). Breast wound healing complications were also higher in the high LAWT groups, although this did not reach statistical significance (P = 0.2). CONCLUSIONS: Abdominal wall thickness is a much better predictor of complications after abdominally based free flaps compared with BMI.