BACKGROUND The recovery of elbow flexion in upper brachial plexus injury can be achieved by the reinnervation of the biceps muscle (single reinnervation), but concomitant restoration of brachialis and biceps function (double reinnervation) has been recommended to improve elbow flexion strength. OBJECTIVE To prospectively compare morbidity and outcomes of single or double muscle reinnervation in restoring elbow flexion following incomplete injury to the brachial plexus. METHODS Forty consecutive patients were prospectively submitted to single or double muscle reinnervation. Elbow flexion strength was evaluated with a push-and-pull dynamometer 12 months after surgery. Hand morbidity related to the procedures was evaluated by the Semmes-Weinstein monofilaments test, quantification of static 2-point discrimination, and measurements of handgrip and lateral pinch strength in serial evaluations up to the final follow-up. RESULTS Similar results for the strength of elbow flexion were observed in both groups. A worsening of sensibility, measured by using Semmes-Weinstein monofilaments, was identified in 8 patients, all of whom showed recovery during follow-up. No worsening of 2-point discrimination was observed. A decrease in handgrip and lateral pinch strength was identified in 8 and 9 patients, respectively, which improved during follow-up. There was no difference between the groups in the incidence of hand motor morbidity parameters. CONCLUSIONS The strength of elbow flexion did not differ significantly between the groups treated with single or double muscle reinnervation. Deterioration of handgrip, lateral pinch strength, and sensibility measured by using Semmes-Weinstein monofilaments, was temporary, resulting in low morbidity for both techniques.