OBJECTIVE: To investigate the functional facilitation effect on an affected side of spastic hemiplegic cerebral palsy by immobilizing the intact side with a cast. METHOD: Five hemiplegic cerebral palsies, who had been treated by general rehabilitation therapy during previous 6 months and showed no functional improvement were studied. Their intact arms were immobilized by scotch short arm casts for 6 weeks. And, the therapeatic effects of this method were evaluated by the Box and Block Test of manual dexterity, Erhardt Developmental Prehension Assesment and Brain Perfusion SPECT. RESULTS: 1. Box and Block test: Before the cast immobilization method, hemiplegic cerebral palsies could transfer a mean of 5.4+/-3.36 cubic rods from one to the other side, and after the treatment a mean of 8.0+/-1.41 cubic rods with the affected arm. 2. Erhardt Developmental Prehension Assesment: Before the treatment, the grasp motion of round rod, cubic rod, and button suited in a mean of 5.1+/-3.0 months, and after the treatment, suited in a mean of 8.2+/-0.66 months. 3. Brain Perfusion SPECTs did not change before and after the cast immobilization method. 4. There was no adverse effect resulting from the cast immobilization method on an intact side. CONCLUSION: We concluded that the cast immobilization method on an intact side could be a new beneficial rehabilitation method for the treatment of spastic hemiplegic cerebral palsies.