Background: Many authors have highlighted the role of muscle strength imbalance around the ankle in the development of recurrent clubfoot following Ponseti treatment. Nevertheless, this possible underlying mechanism behind recurrences has not been investigated sufficiently to date. This study aimed to explore whether there is a relationship between Achilles tendon elongation and recurrent metatarsus adductus deformity in children with unilateral clubfeet treated by Ponseti method. Methods: A retrospective chart review was performed on 20 children (14 boys, 6 girls; mean age: 7 years; age range: 5-9) with a recurrent metatarsus adductus deformity treated by the Ponseti method for unilateral idiopathic clubfoot. At the final follow-up, isometric muscle strength was measured using a portable, hand-held dynamometer in reciprocal muscle groups of the ankle. The length of the tendons around the ankle was ultrasonographically measured. Results: The plantar flexion/dorsiflexion ratio was lower on the involved side (p = 0.001). No significant differences in the strength ratio of inversion/eversion were found (p = 0.4). No difference was observed in lengths of tibialis anterior and posterior tendon (p = 0,1), but Achilles tendon was longer on the involved side (p = 0.001; p < 0.01). A significant negative correlation was discovered between involved/uninvolved Achilles tendon length ratios and involved/uninvolved plantar flexion strength ratios (r = −0.524; p = 0.02) Conclusions: Achilles tendon elongation may be a contributor to the muscle imbalance in clubfeet with the relapsed forefoot adduction treated by the Ponseti technique.