This longitudinal study investigated the long-term effect of different treadmill interventions on the development of gait patterns in infants with Down syndrome (DS). Thirty infants with DS (mean age 10.0 months, S.D. 1.9 months) were randomly assigned to either a "low intensity-generalized" (LG) training group, or a "high intensity-individualized" (HI) training group. Treadmill intervention was conducted in infants' homes until they walked three steps independently. Twenty-five participants completed a 1-year gait follow-up after the treadmill intervention. Six basic gait parameters were examined: normalized velocity, cadence, step length, step width, double support percentage, and dynamic base. Principal component analysis (PCA) conducted for the set of the six basic gait parameters demonstrated that the first principal component (PC1) accounted for 83.8% of the variance. A two-way ANOVA with repeated measures conducted with PC1 scores revealed a significant visit effect and group difference; both groups significantly increased PC1 scores over time, and the HI group produced significantly higher PC1 scores than the LG group. Specifically, the HI group produced significantly higher normalized velocity and cadence, and lower double support percentage than the LG group. In addition, both groups significantly reduced foot rotation asymmetry over time although no difference was found between the two groups. We concluded that the HI treadmill intervention provided a better long-term effect on the development of basic gait parameters than the LG training, and the reduction of foot rotation asymmetry over time was not differentially affected by the different training protocols.