The numerous symptoms and unpredictable pattern of myalgic encephalopathy (ME) make it difficult to describe, especially for children. It was left to carers to guess what the child could achieve each day, often leading to over/underestimates. A functional ability scale was needed, which measured from 0 to 100 percent able and that children and young people themselves designed. A new scale was developed from the Moss Ability Scale using the critique of 251 children and young people from the Association of Young People with ME (AYME). Responding to the shift in emphasis towards patients taking an active role in their own care, it was felt these young people would know whether the scale measured what it had set out to measure, and were asked questions on the face and content validity of the scale. There was a 99 percent agreement between the young people that the final scale was 'workable' or better.