This study examined whether 20-m shuttle-run performance, sex, body mass index (BMI), age, height, and weight are associated with peak oxygen uptake (VO[subscript 2peak]) in youth with Down syndrome (DS; n = 53; 25 women, age 8-20 years) and whether these variables can be used to develop an equation to predict VO[subscript 2peak]. BMI, 20-m shuttle-run performance, and sex were significantly associated with VO[subscript 2peak] in youth with DS, whereas age, height, and weight were not. A regression model included only shuttle-run performance as a significant predictor of VO[subscript 2peak]; however, the developed prediction equation had low individual predictability. Therefore, 20-m shuttle-run performance alone does not provide valid prediction of VO[subscript 2peak] in youth with DS. Sex, BMI, age, height, and weight do not improve the prediction of VO[subscript 2peak]. (Contains 1 figure and 2 tables.)