Classification of the hyperlipoproteinemias permits a rational approach to the diagnosis and management of these conditions. This study included 214 patients with primary hyperlipidemia. The Fredrickson-Lees criteria were used. There were no type 1 cases, 75 type 2, 12 type 3, 71 type 4, 12 type 5, and 44 unclassifiable. Abnormal responses to glucose tolerance tests occurred in 3 of 23 patients with type 2 and 52 of 54 patients with types 3, 4, and 5 disease. Most cases may be classified successfully if the patient's clinical history, fasting serum cholesterol and triglyceride levels, and serum lactescence are known; in this group, the omission of lipoprotein electrophoresis altered classification in no type 2, six type 3, and four type 4 patients.