The role of the medial capsule and transverse metatarsal ligament in hallux valgus deformity including stability of the first metatarsophalangeal and adjacent joints was investigated in vitro. The three-dimensional positions of the proximal phalanx, first metatarsal, and second metatarsal before and after sectioning the medial capsule and metatarsal ligament were measured using a magnetic tracking system. Valgus deformity of the hallux increased with medial capsule sectioning an average of 22.3 degrees +/- 6 degrees. Valgus deformity of the hallux increased with medial capsule and metatarsal ligament sectioning an average of 27.4 degrees +/- 9.1 degrees. Valgus deformity of the hallux did not change significantly after sectioning the metatarsal ligament only. No significant changes were found in varus and eversion of the first metatarsal, in valgus of the second metatarsal, in the distance between first and second metatarsal heads after sectioning the medial capsule, or in the metatarsal ligament. This study shows the importance of the medial capsule in hallux valgus deformity. The transverse ligament did not contribute substantially to cause the deformity.