This study investigated the use of a custom cavus foot orthosis (CFO) in the treatment of ankle instability and pain associated with the subtle cavus foot, a common pathological foot alignment in the United States population. Patients referred by a single orthopedic foot and ankle surgeon to a single pedorthotist for a CFO over a 2-year period were eligible. Pain score pre-and postorthosis and number of instability events pre- and postorthosis were retrospectively evaluated. Ninety-three of 174 eligible patients participated. Average age was 48 years (range, 20-75) and patients suffered a variety of foot pathologies. Average pre-CFO pain score was 7.22 (0 no pain, 10 worst pain). Post-CFO pain score average was 2.41 (p < .0005). Ninety-two percent of patients reporting ankle instability as a problem experienced a decrease in the frequency of instability events post-CFO. The custom cavus foot orthosis is effective at relieving pain and reducing ankle instability in the patient with the subtle cavus foot alignment.