The rate of oxygen uptake can be used to assess energy expenditure during walking, but the necessary instrumentation is cumbersome, expensive, and usually unavailable in the clinical setting. Heart rate is an easily measured parameter, but its use as an index of energy expenditure in children has not been validated previously. We found that the relationship between oxygen uptake and heart rate was linear throughout a wide range of walking speeds for both children with cerebral palsy and normal children. There was no significant difference between the slope or the gamma-intercept of the lines for the two groups. These findings validate the use of heart rate as an index of energy expenditure for normal children and for children with cerebral palsy.