The authors describe a case of retained sterile matrix in a 38-year-old Hispanic man with a history of remote trauma and soft-tissue coverage with a groin flap 13 years before presentation. The patient presented with a slowly enlarging, vertically growing dorsal thumb mass with occasional drainage. The patient had minimal functional impairment, and radiographic imaging showed a radiolucent mass projecting dorsally over the distal phalanx. Surgical exploration revealed an approximately 2 × 2 cm keratinized mass attached to a retained nail bed. The keratinized nail plate was removed, along with an ellipse of soft tissue around the draining tract. To the authors' knowledge, this case is the largest reported vertically growing, retained, and cornified nail bed with an unusual size and shape. Physicians should consider the possibility of retained nail plates in patients who present with unusual large growths after trauma or surgery.