For children and adolescents with cerebral palsy (CP) classified as Gross Motor Function Classification System (GMFCS) level III there is no running-based field test available to assess their cardiorespiratory fitness. The current study investigated whether a shuttle run test can be reliably (test-retest) performed in a group of children with spastic diplegia (eight male, five female) classified as GMFCS level III. Thirteen children (mean age 12y, SD 3y) had to walk/run in squares of 7.5 m delimited by cones. The auditory signals from the GMFCS II compact disc (as used in a previous reliability and validation study) were used during the test, resulting in a starting speed of 1.5 km/hour with a graded increase in speed of 0.19 km/hour per minute (shuttle). Intraclass correlation coefficients (two-way mixed) for achieved shuttles were 0.98. The standard error of measurement was 0.48 levels and the smallest detectable change was 1.32 shuttles. The results are the first indication that the shuttle run test protocol could be reliably performed in this population.