Direct measurement of energy expended by spasticity in children with severe spastic quadriparesis is difficult. Insertion of an intrathecal baclofen pump in a 13-year-old boy with severe spasticity and profound mental retardation resulted in an estimated 30 to 40% decrease in his spasticity. As he had been on a carefully calculated ketogenic diet and fed by gastrostomy, his precise caloric intake was known. Decrease in spasticity, on the same caloric intake, led to marked weight gain. Reduction of 100 calories intake resulted in new weight stability. It was possible therefore, to estimate indirectly energy used by his spasticity. This 100 calories, representing 34% of calories above his resting energy requirement, corresponded to an independently estimated 30 to 40% of caloric expenditure of his spasticity. It was concluded that when calculation of calories is critical, energy utilization by spasticity must be taken into consideration.