The energy cost of locomotion (C) is a useful tool for quantifying the level of walking disability in the clinical evaluation of patients with cerebral palsy (CP). In addition to clinical condition, also age and velocity (v) can influence C, a fact that is often overlooked. To show: i) that C differs in the clinical subtypes of CP (hemiplegia or diplegia) and ii) that C should be measured at comparable speeds in CP patients and controls (of the same age). Controlled study. Pediatric Rehabilitation Unit of "E. Medea" Scientific Institute (Conegliano, TV); Exercise Physiology Lab of University of Verona. Forty-three CP children (32 diplegic: Dg; 11 hemiplegic: Hg) and 20 healthy children (Cg) with an age range of 4-14 years. C was measured as the ratio of net oxygen uptake to walking speed (at v from 1 to 6 km·h(-1)). The Locomotory index (LI) was calculated as the ratio of C in Dg/Hg and Cg (of the same age) at the same speed. C decreases with increasing speed in all groups but evolves differently in Hg and Dg: in the former C decreases by increasing age, becoming similar to that of Cg at 12-14 years; in the latter C does not change as a function of age being always larger than in Cg. Our data highlight the reduction in C with increasing speed and suggest a better prognosis of locomotion for Hg compared to Dg.