A four-month-old infant, thought to suffer from cerebral palsy, presented with respiratory failure on the background of a gradually deteriorating general level of function. Whilst being ventilated in intensive care he was noted to have severe muscle weakness. A disorder of the neuromuscular junction was suspected and he was subsequently demonstrated to have a congenital myasthenic syndrome. Anticholinesterase therapy produced a dramatic recovery. The congenital myasthenic syndromes and the diagnosis of a "floppy baby" are briefly reviewed.