In the rare cases of idiopathic valgus foot that fail to respond to conservative treatment surgery is indicated. The author describes an oblique osteotomy of the calcaneum extending from the pre-Achilles region to the inferior margin of the calcaneo-cuboid joint whereby the whole plantar surface of the calcaneum is rotated around its longitudinal axis. This reduces the sliding plane of the talo-calcaneal joint and changes the direction of the Achilles tendon and tibialis posterior so that they exert a more favourable action on the forefoot and tend to actively stabilise the correction. The results in 9 patients (11 feet) aged from 8 to 10 years are most encouraging and the operation preserves the dynamics of the hindfoot in every aspect. The case material is necessarily small because the indications for surgery in this condition are very rare.