This study explored the effects of a flexible supramalleolar orthosis (SMO), indicated to decrease pronation associated with hypotonia, on postural stability in children with Down syndrome. Seventeen children with Down syndrome (nine males, eight females; mean age 5 years 10 months, SD 17.2 months; range 3 years 6 months to 8 years) were tested three times in a 10-week period (weeks 1, 3, and 10) using the Standing and the Walking, Running, and Jumping dimensions of the Gross Motor Function Measure (GMFM), and the Balance subtest of the Bruininks-Oseretsky Test of Motor Proficiency (BOTMP). Range of motion measurements were used to explore the influence of joint laxity. Significant improvement was found with SMOs compared with shoes only in the Standing dimension (p=0.001) and the Walking, Running, and Jumping dimension (p=0.0001) of the GMFM, both at the time of fitting (week 3) and after 7 weeks of wearing SMOs (week 10). For the BOTMP Balance subtest, significant improvement (p=0.027) was seen only at the end of the 7-week study period. Amount of joint laxity did not influence response to orthotic intervention. This study showed that young children with Down syndrome showed immediate and longer-term (after 7 weeks of use) improvement in postural stability with the use of flexible SMOs.