To determine the effect of diabetic treatment on plasma high density lipoprotein (HDL) composition and lecithin: cholesterol acyltransferase (LCAT) activity, 196 diabetic patients admitted to our hospital to receive diabetic control were divided into three groups according to the type of treatments; insulin treated group (91 patients), sulfonylurea group (56) and diet alone group (49). Plasma lipids, lipoproteins, apolipoproteins and LCAT activity were measured before and after the admission for 2-3 weeks. As compared with the pre-admission, the following results were obtained at the time of discharge in each group. 1) Body weight, fasting blood glucose, triglyceride, total cholesterol, apolipoproteins (A-I, A-II, C-II, C-III, E) were significantly decreased in each group. 2) Apoliprotein A-I/A-II ratio was significantly increased from 4.9 to 5.3 in insulin treated group (p<0.001). 3) Total HDL-cholesterol was significantly decreased in diet alone and sulfonylurea groups, however HDL 3-cholesterol was significantly decreased in sulfonylurea and insulin group. Then HDL 2-cholesterol/HDL 3-cholesterol ratio was significantly increased from 4.8 to 5.1 (p<0.001) in insulin group. 4) LCAT activity were significantly decreased from 790 to 637 (nmol/ml•h) (p<0.05) in insulin group, but did not change in other two groups. In conclusion, compared to diet alone and sulfonylurea groups, insulin group made significant reduction of HDL 3-cholesterol and LCAT activity during hospitalization.