Preexpanded flaps are a method to replace a larger postburn contracture area. We describe the use of a preexpanded vertical trapezius musculocutaneous flap for reconstruction of a severe mentosternal contracture in a 10-year-old male patient with second- to third-degree burn wounds. A 500 cm(3) rectangular-shaped tissue expander was inserted under the trapezius muscle via a lateral incision in first stage. Two months later, after serial expansion of the expander, the neck scar was excised. The preexpanded flap (size = 27 x 9 cm) was harvested and rotated into the defect. The donor site could be closed primarily. The flap totally survived. The hospital stay was 7 days. After 6 months follow-up, a significant improved range of motion with good esthetic outcome was achieved. The use of a preexpanded trapezius flap can provide thin, large, and pliable tissue for reconstruction of a severe mentosternal scar contracture with excellent functional outcomes.