Infants with Down's syndrome are delayed in achieving motor milestones. When they first sit unsupported a discrepancy between visual and mechanical-vestibular indices of postural stability is less disruptive of their balance than in normal infants. Yet when they first stand unsupported, the same discrepancy disrupts balance more in these infants than in the normal infants. The effect of discrepant visual feedback also differs systematically as a function of the infants' experience of the posture. Monitoring posture in relation to a stable visual surround appears to be fundamental to the normal development of motor control.