The purpose of this article is threefold: (1) to describe the development, reliability, and validity of a revised physical performance measure for individuals with mucopolysaccharidosis type I (MPS I); (2) to standardize the test on a normal sample; and (3) to compare results from a selected sample of individuals with MPS I with age-based centiles. The MPS Physical Performance Measure (MPS-PPM) is composed of eight timed functional tasks (FT-8) and two endurance tasks with a modified Energy Expenditure Index for comfortable walking (CW) and fast walking (FW) speeds. Age norms were derived from a convenience sample of 150 typically developing children and adolescents (75 males, 75 females; mean age 11y 2mo [SD 4y 5mo]; range 5–22y). Using a Rasch model for speed tests and confirmatory factor analysis, we established the unidimensionality of the FT-8. Interrater reliability of the FT-8 (intraclass correlation [ICC]=0.98) and test–retest reliability of the FT-8 (ICC=0.96), CW (ICC=0.91), and FW (ICC=0.83) were good. Results of the age-based profiles in 10 individuals with MPS I (five males, five females; mean age 14y 2mo [SD 7y 6mo]; range 6–29y) indicate that the amount of time needed to perform functional tasks is severely affected by the disease, and most individuals were at or below the fifth centile for their age. The patterns of limitations in endurance were more varied. These results suggest the utility of using this revised MPS-PPM to identify the extent of limitation in age-expected physical performance. Implications for using the MPS-PPM for monitoring physical performance changes during clinical interventions are discussed.