The clinical and biochemical features of eleven patients with Type V hyperlipoproteinaemia have been reviewed. All patients were male, and there was a high incidence in the group of obesity, vascular disease, acute abdominal pain, gout, diabetes mellitus and alcoholism. Plasma cholesterol concentrations ranged from 212 to 1512 mg/100ml and triglycerides from 708 to 7670 mg/100 ml. Lipaemia was associated with significant hyponatraemia, and also interfered with the determination of plasma glucose and serum amylase. Chylomicronaemia and hyperprebetalipoproteinaemia were accompanied by reduction in the pools of beta and alpha lipoproteins. All lipoprotein classes were relatively depleted of cholesterol compared to triglyceride. There was a variable pattern of treatment response. In some patients alcohol withdrawal produced a rapid improvement in plasma lipids. In diabetes mellitus there were two types of response: a rapid one in chronic insulin deficiency, and secondly, a more gradual one in mild diabetes associated with hyperinsulinaemia. In other patients there was a rapid response to carbohydrate-calorie restriction but the respective contributions of each of the steps remained unclear.