Of 98 adult-onset diabetic patients selected at random from an ambulatory clinic, 37.8 per cent were found to have hyperlipidemia. In follow-up examination of the hyperlipidemic patients, 23 per cent had hyper-β-lipoproteinemia (Type II), 58 per cent had hyper-pre-β-lipoproteinemia (Type IV), 13 per cent had increased plasma β-and pre-β-lipoproteins (combined type) and 6 per cent had reverted to normal values. Type IV patients, as a group, were distinguished by greater hyperglycemia and relative body weight. Chylomicronemia was not observed in this study group. Hyperlipoproteinemia was frequent in this adult-onset diabetic population and differed in several respects from similar data in untreated juvenile-onset diabetes reported by others.