The present study addressed the question whether the deviant postural adjustments in children with spastic diplegia can be attributed to their crouched sitting position or primarily to their neural deficit. Postural adjustments during sitting in an erect and in a crouched position on a movable platform were assessed in 10 children, aged 3 to 7 years 6 months, with mild-to-severe forms of spastic diplegia and 10 age- and sex-matched control children. Multiple surface EMGs of neck, trunk, and leg muscles and kinematics of head, body sway, and pelvis were recorded during forward and backward translations. The children with normal motor development showed a distinct adaptation of postural adjustments to sitting position. The children with cerebral palsy (CP) had a deficient adaptational capacity which was more pronounced in the erect than in the crouched position. Thus, the crouched sitting position did not induce postural deficiency but seemed to offer a solution to the sensory–motor problem of the instability experienced. Children with severe diplegia exhibited a lack of direction specificity in the leg muscles during backward body sway, which points to a basic deficit in postural control. In addition, these children showed marked dysfunctions in the precise tuning of the postural adjustments to task-specific conditions. In the children with mild-to-moderate forms of CP the basic level of control was intact.