The objective of this study was to evaluate the validity of the Gross Motor Function Measure (GMFM) in children with spastic diplegia. The GMFM is a criterion-referenced evaluative tool designed to quantify change in motor function over time. The GMFM was administered to 37 children with spastic diplegia three times: once as a baseline and again after 12− and 24-month intervals. Paired videotapes of the GMFM assessments were viewed by a therapist familiar with the GMFM, but unfamiliar with the child. The therapist was asked to judge how much change in motor skills was observed between the tapes. The hypothesis was that correlation of change scores on the GMFM and masked therapist's judgment of change by videotaped assessment would be ≥0.45. Spearman correlation coefficients for the GMFM total percent score were 0.66 and 0.79 at 12 and 24 months, respectively. The study provides evidence of the construct validity of the GMFM to measure change in motor function over a 12− and 24-month period of time in children with spastic diplegia.