Botulinum toxin A (BTX-A) is widely used in the management of muscle spasticity in children. However, at present the dose of BTX-A for a given patient is selected empirically. The aim of this study is to provide dosage guidelines that are based on risk/benefit assessment. This was a multicentre retrospective study of the safety profile and efficacy of BTX-A in children with chronic muscle spasticity. Data in 758 patients who received a total of 1594 treatments were analysed (mean age 7.2 years; 429 males, 329 females). Spastic cerebral palsy (CP) was the most common diagnosis (94% of the study sample). Of all treatments 7% resulted in adverse events; incidence was related to the total dose rather than the dose calculated on the basis of body weight. The highest incidence of adverse events was observed in patients who received >1000 IU of BTX-A per treatment session. The odds of an adverse event was 5.1 times greater for this group of patients than for those who had 250 IU or less (p<0.001). A good overall response to treatment was reported in 82% and treatment goals were fully or partially achieved in 3% and 94% of participants respectively. More patients in the highest dose group reported functional deterioration. Interestingly, multilevel treatments resulted in a better response than single-level treatments (odds ratio 1.7, 95% CI 1.3 to 2.2, p=0.001).