The effect of changes in diameter of the flexor sheath after tendon repair on tendon function was studied using the long toes of 64 white leghorn chickens. Biomechanical studies of gliding excursion, toe flexion, and ultimate load were carried out at 6 and 12 weeks after tendon repair to determine functions of the tendons healed in narrowed, directly closed, partially excised, and enlarged flexor sheath experimental groups. Histologic examination was used to evaluate the extent of adhesions, healing status of the tendons, and status of the managed sheath. At 6 weeks, gliding excursions and range of toe flexion were statistically the smallest in the group with sheath narrowing, statistically larger in the groups with direct sheath closure or sheath excision, and statistically the largest in the sheath enlargement group (p < .01). These differences in gliding excursions and range of toe flexion were persistently observed at 12 weeks. The ultimate load of the repaired tendons was statistically the weakest in the group with sheath narrowing at either 6 or 12 weeks (p < .01). However, the ultimate load of the groups with sheath enlargement, direct sheath closure, and partial sheath excision was statistically the same. Results of histologic examination demonstrated more severe adhesions and worsened tendon healing in specimens with sheath narrowing. The repaired tendons in the group with sheath enlargement healed better and had less severe peritendinous adhesions than those in the group with sheath narrowing. The extent of adhesions and tendon healing were similar in the groups with sheath enlargement and direct sheath closure. This study demonstrates that the diameter of the repaired sheath exerts significant influence on flexor tendon function. Enlargement of the digital flexor sheath may provide an additional way to improve function of repaired tendons.