A patient with unusually severe hypertriglyceridaemia (serum concentration initially 258 mmol/l or 22600 mg/dl) and hypercholesterolaemia is reported and discussed. The triglyceride elevation was found to reside within the very low density lipoprotein fraction and was probably attributable to the combination of diabetes mellitus and familial hypertriglyceridaemia. Treatment with insulin and restriction of dietary carbohydrate led to a 50% reduction in the triglyceride concentration, and the addition of nicotinic acid in modest doses led ultimately to a complete normalization of the patient's lipid values. A close correlation was noted between the falling triglyceride concentration and the rising serum sodium concentration during the course of successful therapy. Overall, it is felt likely that this patient's severe and reversible hypertriglyceridaemia was on the basis of excessively rapid lipolysis leading to high concentrations of very low density lipoprotein production. Combined therapy with insulin and nicotinic acid is recommended for other patients of this nature.