Constraint-induced movement therapy (CIMT) is a physi- cal rehabilitation regime that has been previously shown to improve motor function in chronic hemiparetic stroke patients. However, the neural mechanisms supporting rehabilitation-induced motor recovery are poorly under- stood. The goal of this study was to assess motor cortical reorganization after CIMT using functional magnetic resonance imaging (fMRI). In a repeated-measures design, 4 incompletely recovered chronic stroke patients treated with CIMT underwent motor function testing and fMRI. Five age-matched normal subjects were also imaged. A laterality index (LI) was determined from the fMRI data, reflecting the distribution of activation in motor cortices contralateral compared with ipsilateral to the moving hand. Pre-intervention fMRI showed a lower LI during affected hand movement of stroke patients (LI = 0.23 ± 0.07) compared to controls (LI unaffected patient hand = 0.65 ± 0.10; LI dominant normal hand = 0.65 ± 0.11; LI nondominant normal hand = 0.69 ± 0.11; P < 0.05) due to trends toward increased ipsilateral motor cortical activation. Motor function testing showed that patients made significant gains in functional use of the stroke-affected upper extremity (detected by the Motor Activity Log) and significant reductions in motor impair- ment (detected by the Fugl-Meyer Stroke Scale and the Wolf Motor Function Test) immediately after CIMT, and these effects persisted at 6-month follow-up. The behav- ioral effects of CIMT were associated with a trend toward a reduced LI from pre-intervention to immediately post- intervention (LI = –0.01 ± 0.06; P = 0.077) and 6 months post-intervention (LI = –0.03 ± 0.15). Stroke-affected hand movement was not accompanied by mirror move-