A 21-year-old woman underwent bilateral augmentation mammoplasty by injection of an unknown volume of fat obtained via trochanteric and abdominal liposuction. The procedure was performed by a surgeon untrained in plastic surgery, at a clinic not affiliated with a hospital. Six months later, she presented to our clinic with a palpable left breast lump. Physical examination revealed a large firm mass occupying the entire upper outer quadrant of the left breast and a normal right breast. Breast ultrasound showed a large, well-defined isoechoic mass in the left upper outer breast. Considering her age, the patient underwent core needle biopsy, since the mass mimicked a phyllodes tumor on ultrasonography. The 14-gauge core needle biopsy demonstrated multiple lipid droplets with some white-yellowish tissue, caused by liquefaction of the injected fat. Histopathologic examination demonstrated the presence of a pseudocyst with fat necrosis, granulomatous reaction to lipid material, and cystic formations containing oily fluid. No signs of malignancy were detected. Surgical excision was performed, and histopathology revealed findings consistent with fat necrosis.