Evidence is increasing to suggest that botulinum toxin type A (BTX-A) plays a role in the management of upper extremity spasticity in the paediatric population. However, little information is available on the clinical characteristics of the child that predict a response to this intervention. Our research group previously published a randomized controlled trial demonstrating that BTX-A injection improves function of the upper extremity of children with spastic hemiplegia. In the present paper, we evaluate the child characteristics that predict a positive response to the BTX-A injections in the randomized treatment group. The treatment group was divided into positive functional responders and nonresponders using a cut score of a change of 10 points on the Quality of Upper Extremity Skills Test (QUEST). A two-way analysis of variance procedure was done comparing the following baseline characteristics: function as scored on the QUEST and the Pediatric Evaluation of Disability Inventory (PEDI) self-care domain, grip strength, upper extremity spasticity and age. Grip strength was significantly higher in responders with a P-value of 0.001. Young age approached significance with a P-value of 0.05. Correlation of change scores on the QUEST with baseline characteristics in the treatment group yielded similar results. BTX-A causes a reduction in spasticity and strength; we postulate that if the hand is weak initially, BTX-A can decrease hand function. Two case reports are presented that highlight the importance of grip strength and age.