The cross-sectional areas of the peroneal and anterior muscle compartments at the same level in the upper leg were measured using magnetic resonance imaging in 41 cases of forefoot pes cavus. The pes cavus group included idiopathic cases and pes cavus associated with Charcot-Marie-Tooth disease, Friedreich's ataxia, cerebral palsy, status postpoliomyelitis, nerve trauma, and spinal cord tethering. Thirty-nine of these cases were symptomatic. The results were compared with studies of 11 normal controls. It was found that in the majority of cases of forefoot cavus, the peroneal compartment was enlarged relative to the anterior compartment when compared with the normal controls. Biopsies of the tibialis anterior and peroneus longus muscles in 18 patients with forefoot pes cavus showed that any relative expansion of the peroneus longus was not due to pseudohypertrophy. Overaction of the peroneus longus in comparison to its antagonist the tibialis anterior is proposed as an important factor in the pathogenesis of the majority of symptomatic cases of forefoot pes cavus.