It has been a half century since MacQuarrie called attention to the serious issues of paediatric hypoglycaemia with his presidential address to the American Pediatric Society on “idiopathic hypoglycaemia of infancy”.¹ He pointed out that “idiopathic hypoglycaemia” could lead to seizures or permanent brain damage, and that it might have a genetic basis; however, he speculated that it was unlikely to involve insulin since insulinomas rarely occur in childhood. Over the years, there have been two types of reactions to MacQuarrie’s formulation of “idiopathic hypoglycaemia of infancy”. All too often it has been the ostrich approach of assuming that “idiopathic hypoglycaemia” is a specific entity, forgetting that “idiopathic” is the medicalese equivalent of “I dunno!”. The second type of response has been to search for underlying causes in children labelled as “idiopathic hypoglycaemia”. The latter approach has been much more rewarding, yielding an ever expanding list of new disorders, including genetic defects of hepatic glucose production (for example, deficiencies of fructose-1,6-diphosphatase, glycogen synthase, and the GLUT1 and GLUT2 transporters); a baker’s dozen defects in ketone synthesis and utilisation (for example, deficiencies of medium chain acyl-CoA dehydrogenase, plasma membrane carnitine transporter, and HMG-CoA lyase and synthase); and congenital glycosylation disorders. In addition, …