Aim: To determine whether the bone mass of young people with Down syndrome may increase, following a 21-week conditioning training programme including plyometric jumps. Method: Twenty-eight participants with Down syndrome (13 females, 15 males) aged 10 to 19 years were divided into exercise (DS-E; n = 14; eight females, six males mean age 13y 8mo, SD 2y 6mo) and non-exercise (DS-NE; n = 14; five females, nine males mean age 15y 5mo, SD 2y 6mo) groups. Total and regional (hip and lumbar spine [L1-L4]) bone mineral content (BMC) and total lean mass were assessed by dual energy X-ray absorptiometry at baseline and after a 25-minute training session performed twice a week. Repeated-measures analyses of variation were applied to test differences between pre- and posttraining values for BMC and total lean mass. Differences between increments were studied with the Student's "t"-test. Linear regression models were fitted to test independent relationships. Results: After the intervention, higher increments in total and hip BMC, and total lean mass, were observed in the DS-E group (all p less than 0.05). A time x exercise interaction was found for total lean mass (p less than 0.05). The increment in total lean mass, height, and Tanner stage accounted for almost for 60% in the increment in total BMC in the DS-NE group (p less than 0.05). Interpretation: Twenty-one weeks of training have a positive effect on the acquisition of bone mass in young people with Down syndrome.