The aim of this study was to examine the responsiveness of the Test of Basic Motor Skills for Children with Down Syndrome (BMS). Forty-one children with Down Syndrome, 3 to 36 months of age, participated in the study. Gross motor skills were assessed three times using the BMS and the Gross Motor Function Measure (GMFM) before and after a baseline period of 2 weeks (T1-T2) and after a period of 16 weeks (T2-T3). Internal and external responsiveness of the BMS was analyzed using Guyatt's Responsiveness Index (GRI) and 2 x 2 repeated measures. Change in BMS scores was compared to change in GMFM scores and parent and physiotherapist ratings of change. The responsiveness of the BMS was large (GRI = 2.55). A significant Time x Age interaction [F(1,37) = 8.87, p less than 0.01] indicated that BMS scores increased more for children [less than or equal] 2 years compared with children 2 to 3 years of age. The difference in GRI between the BMS and GMFM was 0.75 (95% CI -0.25-1.75), indicating that the BMS and GMFM did not significantly differ in responsiveness. The Time x Measure interaction was not significant, indicating that the increase in mean BMS and GMFM scores did not differ. Change in BMS scores were correlated with parent ratings of change (r = 0.65, p less than 0.001) but not physiotherapist ratings (r = 0.36, p = 0.23). The results provide evidence that the BMS is responsive to change in gross motor development in children with Down Syndrome from 3 to 36 months of age. (Contains 5 tables and 2 figures.)