Diabetic lipaemia, severe hypertriglyceridaemia associated with diabetic ketoacidosis, is a well recognised, but rare condition. Why this occurs in some patients and not others is unknown. We report a case of extreme lipaemia in a 20-year-old woman with type 1 diabetes who presented to hospital with diabetic ketoacidosis (DKA). At admission the patient's blood was grossly lipaemic and plasma lipid analyses showed triglyceride and cholesterol concentrations of 379 mmol/L and 52 mmol/L, respectively. She had no peripheral stigmata of chronic hyperlipidaemia and 1 year previously her plasma triglyceride and total cholesterol concentrations were 2.5 mmol/L and 4.4 mmol/L respectively. She was treated with insulin and the hypertriglyceridaemia resolved over several days. Because of the marked hypertriglyceridaemia, lipoprotein lipase (LPL) genetic testing was performed. Sequencing of the LPL gene revealed that she was heterozygous for the common S447X LPL variant and heterozygous for a novel missense mutation in exon five (I225N). Ile225 is highly conserved among species and this mutation is predicted to impair function of the mature LPL protein. We conclude that heterozygosity for LPL mutations may predispose to transient severe hypertriglyceridaemia, when combined with insulin deficiency.