The objective of this study was to determine whether the use of intramuscular botulinum toxin A (BTX-A) increases upper limb function and skills in the context of a specific therapy programme in children with hemiparetic cerebral palsy. Twenty children (nine females, 11 males) aged 4 to 16 years who were thought likely to benefit from BTX-A treatment were included. After matched pairs were made, on the basis of Zancolli grade and age, randomization took place. All patients were given structured rehabilitation (physiotherapy and occupational therapy three times a week for 6 months), and half of the patients received intramuscular BTX-A. No placebo injections were given in the control group. Participants were assessed at baseline, at 2 and 6 weeks, and at 3, 6, and 9 months after injection. The Ashworth scale, active range of motion of arm joints, the Melbourne assessment of upper limb function, the Pediatric Evaluation of Disability Inventory, and the nine-hole peg test were used for outcome measurement. Observers were blinded for treatment allocation only for scoring the Melbourne test. The children in the treatment group showed a clinically relevant increase in active dorsal flexion, and tone reduction of the wrist. For the functional outcome measures, no statistically significant differences between the groups could be demonstrated. Intramuscular BTX-A added to an intensive therapy programme reduces impairment for at least 9 months; the effect on activity level is still uncertain.