An 8-year-old boy with a history of the inability to extend the middle finger at the proximal interphalangeal and distal interphalangeal joints with the wrist and metacarpophalangeal joints in the neutral position since the age of 5 months had surgery. The flexor digitorum profundus muscle of the middle finger had a short muscle belly and was replaced in part with mature fat tissue. Histologic examination of the resected tissue showed replacement of a part of the muscle with mature fat tissue. It was speculated that some myosatellite cells, which typically differentiate into skeletal muscle cells, may have erroneously differentiated into adipocytes in the first postnatal year, during which complete development of skeletal muscles occurs.