Ipsilateral peroneus brevis muscle histopathology was studied in 64 children with idiopathic rigid equinovarus at the setting of initial posteromedial-lateral release. Fifty percent of biopsies demonstrated abnormal muscle fiber morphology, classified as congenital fiber-type disproportion or fiber-size variation. Forty-one infants (59 feet) underwent initial surgical intervention within the first year of life with a minimum 2-year follow-up. Feet with abnormal muscle histology had a significantly greater incidence of recurrent equinovarus deformity requiring reoperation; the relative risk of clubfoot recurrence in children with fiber abnormalities was 5.6. Male patients with bilateral deformity and abnormal peroneus brevis histology had a particularly high incidence of recurrent equinovarus. Developmental internal tibial torsion requiring surgical intervention was also greater in the abnormal-fiber histology group. The incidence of postoperative metatarsus adductus/varus necessitating surgery was comparable despite histologic findings. Thus muscle-fiber abnormalities are prevalent in idiopathic equinovarus. Such fiber-type anomalies may predict recurrent limb deformities.