Objective To investigate the relationship of white matter integrity and path of the corticospinal tract (CST) on arm function before and after constraint-induced (CI) movement therapy in children with hemiparetic cerebral palsy (CP) and adults with chronic stroke. Design Study 1 used a multiple-baseline pre-post design. Study 2 was a randomized controlled trial. Setting Outpatient rehabilitation laboratory. Participants Study 1 included children with hemiparetic CP (n=10; mean age ± SD, 3.2±1.7y). Study 2 included adults with chronic stroke (n=26; mean age ± SD, 65.4±13.6y) who received either CI therapy or a comparison therapy. Interventions Children in study 1 received CI therapy for 3.5h/d for 15 consecutive weekdays. Adults in study 2 received either CI therapy or a comparison therapy for 3.5h/d for 10 consecutive weekdays. Main Outcome Measures Diffusion tensor imaging was performed to quantify white matter integrity. Motor ability was assessed in children using the Pediatric Motor Activity Log-Revised and Pediatric Arm Function Test, and in adults with the Motor Activity Log and Wolf Motor Function Test. Results Participants in both studies improved in real-world arm function and motor capacity. Children and adults with disrupted/displaced CSTs and children with reduced fractional anisotropy values were worse on pretreatment tests of motor function than participants with unaltered CSTs. However, neither integrity (fractional anisotropy) nor distorted or disrupted path of the CST affected motor improvement after treatment. Conclusions Participants who had reduced integrity, displacement, or interruption of their CST performed worse on pretreatment motor testing. However, this had no effect on their ability to benefit from CI therapy. The results for children and adults are consistent with one another. © 2014 by the American Congress of Rehabilitation Medicine.