The clinical characteristics of clubfoot suggest a relative hypoplasia of the posterior and medial foot and leg. To test this hypothesis we compared quantitative cell and organelle characteristics between anterior and posterior tibial tendon sheaths from four clubfeet and two normal feet. Clubfoot posterior tibial tendon sheath had significantly less cell and cytoplasm volume than did clubfoot anterior sheath. Normal posterior tibial tendon sheath had significantly greater cell and cytoplasm volume than did normal anterior sheath. A similar tendency was found for nuclear volume and cell number, although not all samples reached statistical significance. No difference in organelle characteristics between anterior and posterior sheaths was found. We interpret this morphologic difference as suggesting a relative inactivity of clubfoot posterior tibial tendon sheath cells. This supports the hypothesis of a regional growth disturbance as the cause of clubfoot.