Autism is characterised by keen interests and differences in social interactions and communication. Activities that help autistic children and young people with social skills are commonly used in UK schools. LEGO ® based therapy is a new activity that provides interesting and fun social opportunities for children and young people and involves building LEGO ® models together. This study looked at LEGO ® based therapy for the social skills of autistic children and young people in schools. It was a randomised controlled trial, meaning each school was randomly chosen (like flipping a coin) to either run LEGO ® based therapy groups in school over 12 weeks and have usual support from school or other professionals, or only have usual support from school or other professionals. The effect of the LEGO ® based therapy groups was measured by asking children and young people, their parents/guardians, and a teacher at school in both arms of the study to complete some questionnaires. The main objective was to see if the teacher's questionnaire answers about the children and young people's social skills changed between their first and second completions. The social skills of participants in the LEGO ® based therapy groups were found to have improved in a small way when compared to usual support only. The study also found that LEGO ® based therapy was not very costly for schools to run and parents/guardians and teachers said they thought it was good for their children and young people. We suggest further research into different potential benefits of LEGO ® based therapy.