A healthy 36-year-old man presented to the orthopedic clinic with an enlarging lower back mass. He had first noticed the mass 3 weeks earlier, after he slipped on the stairs and slid down on his back. On physical examination, a large, soft, fluctuant mass with no overlying skin necrosis was palpated on the lower back (Panel A). Magnetic resonance imaging of the lumbosacral spine showed an encapsulated, subcutaneous, suprafascial lesion, measuring 20 cm by 20 cm by 10 cm, that extended from the lower lumbar area to the buttocks (Panel B), a finding consistent with a Morel–Lavallée lesion. A Morel–Lavallée lesion is a traumatic, closed degloving injury that results from shearing of subcutaneous tissue away from the underlying fascia. Its management varies; in this case, surgical excision was performed because of the risk of recurrence, given its encapsulation and chronicity. Histologic examination of the excised cystic mass showed a pseudocyst lined with myofibroblasts. Three months after the surgery, the patient had recovered fully without recurrence of the mass.