Acute extensor carpi ulnaris (ECU) subsheath injury and chronic subsheath insufficiency may result in symptomatic ECU instability at the level of the distal ulna osseous sulcus. Associated ulnocarpal (ie, triangular fibrocartilage complex) and ECU intrinsic tendinopathic changes may accompany subsheath pathologies and require concomitant treatment. Surgical treatment is indicated in refractory cases despite nonoperative treatment and may consist of repair of the torn edge of the ECU subsheath or, more frequently, reconstruction utilizing a radially based extensor retinacular sling. An ECU subsheath reconstructive technique is detailed.