We report the case of a young boy carrying a de novo missense mutation (c.1199G>T; p.R400L) in the SLC2A1 gene who presented initially with benign myoclonic epilepsy of infancy. Eventually, he had a poor outcome with refractory generalised tonic-clonic, myoclonic and absence seizures, ataxia, significant mental impairment and slowing of head growth. He responded poorly to ketogenic diet. This case extends the phenotype of GLUT1-related syndromes and also sheds light on the genetic basis of myoclonic epilepsies of infancy suggesting that variable outcome may depend on genetic factors. [Published with video sequences].