Objective: To assess the efficacy of transcranial direct current stimulation (tDCS) on decreasing upper-limb (UL) muscle tone after stroke. Design: A prospective, sham-controlled, randomized controlled trial with 4-weeks follow-up. Randomization into the tDCS group or the control group. Setting: Rehabilitation education and research hospital. Participants: Inpatients (N=90, 45 per group; age range, 15-70y; 69 men, 21 women; duration of stroke, 2-12mo) with poststroke UL spasticity. No participant withdrew because of adverse effects. Intervention: The tDCS group received tDCS to the primary sensorimotor cortex of the affected side with cathodal stimulation, 20 minutes per day, 5 days per week, for 4 weeks and conventional physical therapy. The control group received sham stimulation (same area as the tDCS group) and conventional physical therapy. Main Outcome Measures: Modified Ashworth scale (MAS), Fugl-Meyer Assessment of motor recovery, and Barthel Index. All outcomes were measured at admission, after treatment, and after follow-up. A clinically important difference (CID) was defined as a reduction of >= 1 in the MAS score. Results: Compared with the sham tDCS group, the active tDCS group had significantly more patients with a clinically important difference after treatment (80% and 78% vs 6% and 9%) and at 4-week follow-up (84% and 82% vs 7% and 4%), and UL motor function and activities of daily living (ADL) assessment improved more significantly in the active tDCS group (Fugl-Meyer Assessment of motor recovery from 12 [range, 4-26] to 22 [range, 7-50] to 32 [range, 28-41], Barthel Index from 55 [range, 0-85] to 85 [range, 5-100] to 90 [range, 10-100 vs Fugl-Meyer Assessment of motor recovery from 8 [range, 3-34] to 10 [range, 8-25] to 15 [range, 6-40], Barthel Index from 55 [range, 25-95] to 65 [range, 30-100] to 75 [range, 40-100], respectively, P<.01). Conclusions: UL muscle tone after stroke can be decreased using cathodal tDCS. Combined with conventional physical therapy, tDCS appears to improve motor function and ADL. Cathodal tDCS over ipsilesional primary sensorimotor cortex may inhibit primary sensorimotor cortex hyperactivation, resulting in significant reductions in muscle tone. Archives of Physical Medicine and Rehabilitation 2013;94:1-8 (C) 2013 by the American Congress of Rehabilitation Medicine