The aim of this study was to assess whether the Energy Expenditure Index (EEI), based on heart rate (HR) adjusted for speed, was a clinical indicator of the Oxygen Consumption Index (OCI), oxygen consumption (VO2) adjusted for speed, during self-paced walking in children with spastic diplegia cerebral palsy (CP) and children without physical disabilities (ND). Ten children with spastic diplegia CP and 15 children who are ND participated. VO2 and HR were measured at rest and during self-paced ambulation using an AeroSport KB1-C portable metabolic system, and the EEI and OCI were calculated. A fair to good correlation (r = 0.61) between EEI and OCI was found for children with spastic diplegia CP ambulating at self-paced speeds while only a moderate correlation (r = 0.40) was found for children who are ND. The results suggest that the EEI may be a useful clinical indicator of OCI at self-paced ambulation speeds in children with spastic diplegia CP and warrants further investigation.