Breast reconstruction in underweight patients (BMI <
18.5 kg/m²) poses unique challenges due to limited subcutaneous fat and higher risks of unnatural contours with implant-based methods. This study evaluated the efficacy of fat-augmented latissimus dorsi (FALD) muscle flaps in addressing these challenges. We retrospectively analyzed outcomes of 12 underweight and 38 normal-weight patients undergoing unilateral breast reconstruction with FALD muscle flaps. Data collection included patient demographics, operative details, fat grafting volumes, complications, and patient-reported outcomes using BREAST-Q. The underweight group required additional fat grafting more frequently (75% vs. 63%, p = 0.003). Despite smaller flap weights (140 g vs. 165 g, p = 0.08) and reduced fat graft volumes (135 ml vs. 186 ml, p = 0.06), both groups achieved comparable aesthetic outcomes. Postoperative complications, including seroma and oil cysts, were similar between groups. While additional fat grafting sessions were more commonly needed in the underweight group, the final aesthetic results, evaluated by surgeon ratings and patient satisfaction, were equivalent to those in the normal-weight group, with high scores observed across all BREAST-Q domains. This study demonstrates that the FALD muscle flap is an effective and adaptable technique for autologous breast reconstruction in underweight patients. By utilizing available fat deposits and adapting to anatomical constraints, this method achieves both functional and aesthetic success. Further research with larger cohorts is needed to confirm these findings and refine this approach for broader application.