OBJECTIVE This study aimed to assess short-term effects of botulinum toxin A in ambulant adults with spastic cerebral palsy. DESIGN A single-centre double-blind, placebo-controlled, randomized clinical trial. SUBJECTS Patients were recruited through advertisements. Inclusion criteria were: spastic cerebral palsy, age 18-65 years, decreased walking, walking without aids for minimum 20 m, and no cognitive impairments. METHODS A total of 66 participants, mean age 37 (standard deviation 11.4) years, were enrolled and received injections of either botulinum toxin A (n=33) or placebo (n=33). Primary outcomes were: sagittal kinematics of ankle, knee and hip, and health-related quality of life (Short Form 36). Secondary outcomes were: visual analogue scale for muscle-stiffness/spasticity, Timed Up and Go, 6-minute walk test, and Global Scale of perceived effect. RESULTS No significant differences were found between the groups in the primary outcomes. In the secondary outcomes the botulinum toxin A group rated improvement in visual analogue scale muscle-stiffness/spasticity and the Global Scale of perceived effect. No serious adverse events occurred. CONCLUSION Botulinum toxin A injections alone gave no benefit over placebo in lower limb sagittal kinematics and Short Form 36 in ambulatory adults with cerebral palsy. However, self-reported rating of muscle-stiffness/spasticity and global effects indicated positive effects of botulinum toxin A. Further studies with specific post-injection rehabilitation and longer study period are warranted.