We report on a 37-year-old woman with a left clubfoot with a progressive decrease in ambulatory distance resulting from fatigue of her left calf muscles. She had multiple surgeries for correction of the clubfoot deformity in early childhood and uses an ankle-foot orthosis (AFO) during gait. Physical examination revealed a decrease in left calf girth. T1-weighted spin echo magnetic resonance imaging (MRI) of her legs distal to the knees showed a marked decrease in leg muscle volume in the clubfoot limb. There was increased subcutaneous fat overlying the leg muscles in the clubfoot limb, and the muscles of the clubfoot limb were infiltrated with fatty tissue. The extent of atrophy of the leg muscles on MRI was significantly greater than expected from the clinical examination. In conclusion, MRI provides a true measure of leg muscle volume in the adult clubfoot and offers an explanation for leg muscle fatigue during ambulation.