The etiology of idiopathic and familial clubfoot remains elusive. The purpose of this study was to document the prevalence of the flexor digitorum accessorius longus muscle in a large series of children treated with an extensive soft tissue release for idiopathic clubfoot deformity and compare those patients with and without this anomalous muscle in terms of various demographic data, including a positive family history of clubfoot. A retrospective review was conducted to identify infants treated with an extensive soft tissue release for idiopathic clubfoot deformity at the authors' institutions between 1980 and 2000. Patient characteristics and family demographic data were analyzed using multiple logistic regression analysis modeling the probability of having the anomalous flexor muscle. A flexor digitorum accessorius longus muscle was identified in 55 (6.6%) of the 835 patients at the time of surgical correction of the clubfoot deformity. It was present in 4.5% of patients without a family history (33/741) and 23.4% of patients with a family history (22/94) (P < 0.0001). Children with first-degree relatives with clubfoot are 6.6 times more likely to have the anomalous flexor muscle than children without first-degree relatives with clubfoot (odds ratio 6.6; 95% confidence interval 3.63-11.84; P < 0.0001).