BACKGROUND The reduction in plasma cholesterol with increase in large and lower dense LDL (pattern A) obtained by statins is usually associated with a prompt reduction in cardiovascular risk, but after bariatric surgery for morbid obesity a delay of some years is observed. No data regarding LDL pattern are available in obese subjects after biliopancreatic surgery. OBJECTIVE To evaluate the modifications in LDL composition and LDL density after biliopancreatic surgery. METHODS 29 patients (17 type 2 diabetics (type 2) and 12 non-diabetics (ND)) with BMI <35, who failed previous attempts to decrease weight by diet, were studied before and 6 months after biliopancreatic diversion for morbid obesity. METHODS In all subjects, besides fasting circulating lipids, glucose and insulin, LDL and VLDL composition were determined and LDL density was evaluated as well. RESULTS After surgery we observed a significant reduction of all circulating lipids, including apolipoprotein (Apo) B. The decrease was more marked for total cholesterol (-41%) than for triglycerides (-28%), without a significant difference between type 2 and ND. After surgery, LDL presented a marked decrease in the percentage of cholesterol (from 36 to 32%) with a marked increase in the percentage of triglycerides (from 13 to 18%), without appreciable modification of ApoB. After surgery, 1 patient changed from pattern B to A, while 2 patients previously pattern A became pattern B. Also a decrease in HDL and ApoAI was evident in all the subjects with an increase in the VLDL-1. CONCLUSIONS Our data indicate that after biliopancreatic diversion, the plasma lipid profile improves along with improvement of plasma glucose and insulin sensitivity, but the LDLs become richer in triglycerides. It is possible that the greater atherogenicity of these LDLs is compensated by an improvement in the general metabolic condition.