Complications following an extensor indicis proprius (EIP) tendon transfer are loss of strength, independence and mobility in index extension. The main goal of this study was to measure independent and dependent index extension strength after a tendon transfer. Secondary goals were to determine if the index kept independent extension after the transfer and to evaluate 2nd metacarpophalangeal joint (MCPJ) mobility. Our series consisted of 19 patients in whom the EIP tendon was divided proximally to the extensor hood. The EIP tendon was retrieved through a proximal approach at the distal radius level and rerouted towards a recipient tendon. At an average follow-up of 41 months, the average independent extension strength was 5.6N for 11N on the contralateral side and the dependent strength was 10.9N for 20N. No patient complained of a loss of extension strength and all had kept an independent active extension on the operated index. The 2nd MCPJ on the operated side had an independent extension lag of 17.9° for 2.6° on the contralateral side and a dependent extension lag of 10.3° for 10.5°. Two patients were impaired in their daily activities when moving the operated index. Our results show that EIP procurement for tendon transfer leads to decreased independent and dependent strength as well as decreased active extension mobility of the 2nd MCPJ. However, the functional impact was negligible and should not compromise the use of the EIP as a tendon transfer.