Serum concentration of apoprotein A, high density lipoprotein (HDL)-cholesterol and HDL-phospholipids has been studied in thirteen consecutive episodes of diabetic ketoacidosis. In three patients with type I diabetes mellitus HDL2 and HDL3 subfractions were also measured. Patients with type I diabetes showed greatly decreased HDL-cholesterol concentration on admission which increased into the normal range after insulin treatment, while HDL-phospholipids decreased during treatment and apoprotein A remained almost unmodified. In three patients with type I diabetes a virtual absence of HDL2-cholesterol subfraction was observed, which rose to normal values during recovery. Conversely, in type II diabetes mellitus HDL-cholesterol was slightly reduced on admission, and tended to decrease during recovery. These findings imply the existence of abnormalities in the qualitative composition of HDL, and indicate that HDL-cholesterol can fluctuate much more rapidly than previously thought.