A recent randomized controlled trial indicated that modified Constraint-Induced Movement Therapy followed by Bimanual Training (mCIMT-BiT) is an effective intervention to improve spontaneous use of the affected upper limb in children with unilateral spastic cerebral palsy (CP). The present study aimed to investigate how the above-mentioned improvements as a result of 8 weeks mCIMT-BiT were established. 52 children with unilateral spastic CP with Manual Ability Classification System (MACS) scores I, II or III and aged 2.5-8 years were randomly allocated to either mCIMT-BiT (n=28) or Usual Care (UC) (n=24). Developmental disregard ("learned non-use") and upper limb capacity and performance scores were derived from the Video Observations Aarts and Aarts, module Determine Developmental Disregard. Active and passive range of motion at the affected wrist and elbow were assessed using goniometry during isolated movements. Upper limb capacity and performance demonstrated significantly greater improvements after mCIMT-BiT compared to UC, which lasted up to 8 weeks follow-up, whereas developmental disregard and passive and active range of motion did not show differential effects. The results support the notion that improvement of capacity and performance of the upper limb through mCIMT-BiT in children with unilateral spastic CP is based on a better utilization of existing motor functions of the affected arm and hand. However, enhancement of the overall amount of use (or the reduction of learned non-use) may still be suboptimal leaving room for improvement of this treatment. (Contains 5 tables and 3 figures.)