Biofeedback of muscle activity is commonly used as an adjunct to physical therapy, but it has not previously been used for long-term treatment of movement disorders. The authors hypothesized that chronic daily use of biofeedback of muscle electrical activity might promote improved use of the upper extremity in children with cerebral palsy and upper extremity motor deficits. They constructed a portable electromyography (EMG) unit that includes a surface EMG sensor and amplifier, microcontroller-based nonlinear signal processing, and vibration feedback of muscle activity. A total of 11 children ages 6 to 16 years, with cerebral palsy or acquired static brain injury, wore the device at least 5 hours per day for 1 month. Changes in upper extremity function were assessed using an individualized Goal Attainment Scale. Results showed significant clinical improvement in all 10 children who completed the study. These results suggest that further testing of prolonged surface EMG biofeedback is warranted.