BACKGROUND: Apolipoprotein (apo) CIII plays an important role in the catabolism of triglyceride-rich lipoproteins as it is a potent inhibitor of lipoprotein lipase (LPL). A low LPL activity has been simultaneously associated with hypertriglyceridemia, low HDL cholesterol and with small LDL particles. AIM: To compare the effects of a 12-week treatment with micronized fenofibrate (200 mg) versus atorvastatin (10 mg) on apo CIII and lipoprotein-lipid levels including LDL size. METHOD: After a 4-week washout period, dyslipidemic patients were randomized to either micronized fenofibrate (n = 64) or atorvastatin (n = 72). RESULTS: Both fenofibrate and atorvastatin significantly decreased apo CIII levels by -0.03 +/- 0.03 versus -0.01 +/- 0.03 g/l respectively, and increased LDL size by 4.9 +/- 3.3 versus 1.8 +/- 2.9 A. Improvements in these parameters were significantly greater with fenofibrate (P < 0.0001). Significant relationships were found between changes in triglycerides and changes in apo CIII (r = 0.81 and r = 0.59, P < 0.0001) as well as between changes in LDL size and changes in apo CIII (r = -0.41 and r = -0.45, P < 0.001), in both fenofibrate and atorvastatin groups. respectively. CONCLUSION: The substantial reduction in apo CIII induced by micronized fenofibrate plays an important role in the greater effect of micronized fenofibrate than atorvastatin on plasma triglycerides and LDL size.