This study compared the energy cost of walking in children with cerebral palsy (CP) classified at different levels of the Gross Motor Function Classification System (GMFCS) with that in children with typical development. Sixteen female and 14 male children with CP (mean age 9 years 6 months, SD 2 years 4 months, range 6 years 4 months to 13 years 4 months) and 14 male and 13 female typically developing children (mean age 10 years, SD 1 year 6 months, range 7 years 1 month to 12 years 11 months) participated. Children with CP were classified at GMFCS level I, n=5; level II, n=10; level II, n=9; and level IV, n=6. Energy cost was assessed by the gas dilution method as each child walked around an oval track wearing a dilution mask. Significant differences were found across GMFCS levels (p<0.0001) and between adjacent levels (p<0.013). Children with CP displayed a higher energy cost of walking than the typically developing children (p<0.0001). A strong correlation (0.87) was found between the energy cost of walking and GMFCS level (p<0.01) when children with typical development were assigned a GMFCS level of zero to allow statistical analysis. This indicates increasing energy cost of walking with increasing severity of functional involvement. These differences in energy cost across GMFCS levels provide another distinguishing factor between GMFCS levels and further emphasize the importance of considering metabolic demand in determining treatment options.