The concept of the differential diagnosis has a central place in all clinical medicine. In neurology and paediatrics, evolution of thought concerning the differential diagnosis of the child presenting with symptoms of neuromuscular disease was far in advance of similar diagnostic approaches to problems of infectious, infective and neoplastic disease; and as such forms a significant historical model for the development of modern clinical approaches to the sick or disabled child. The account presented in this paper provides a detailed historical review of the development of thought relating to the causes of neuromuscular disease. Nosological developments concerning diseases of the motor unit can be conveniently classified into three periods: (a) a "state of the art" period to 1850; (b) 1850-1890 - a period of parallel and interdependent advances in both normal neuro-anatomy and neurophysiology, and diseases recognized as variations from such norms. In this era Duchenne was the first to write about the differential diagnosis of the progressive muscular paralysis in childhood; and Gowers was the first to write specifically on the hereditary transmission of this group of diseases; (c) in 1891 was described the first case of childhood neuronopathy recognized as such, and with this development was ushered in the modern era of clinical differential diagnosis of childhood neuromuscular disease. The "splitters" have won the great debate concerning the clinical approach to neurological diseases. The continual further refining of an exact diagnosis is the only way in which a realistic prognosis can be forecast, correct genetic counseling can be offered, and (if the condition is treatable) optimal therapy can be introduced.