Plasma low density lipoprotein (LDL) plays a central role in atherogenesis, and elevated levels of LDL are associated with an increased risk of coronary heart disease (CHD). Studies have now revealed that LDL is structurally heterogeneous, based on its size and density. Patients with combined hyperlipidemia exhibit a lipid profile - the so-called atherogenic lipoprotein phenotype - that is associated with elevated triglyceride levels, low levels of high density lipoprotein and a preponderance of atherogenic, small, dense LDL particles. Such individuals are at an increased risk of CHD events, regardless of their total LDL circulating mass. Evidence suggests that when plasma triglycerides exceed a critical threshold of approximately 133 mg/dl (1.5 mmol/l), this favours the formation of small, dense LDL from larger, less dense species. Lipid-lowering agents that are capable of lowering triglyceride levels below this threshold value will cause a shift to a less dense and, therefore, less atherogenic LDL profile. This effect has been demonstrated for the HMG-CoA reductase inhibitor atorvastatin which, in addition to its ability to markedly decrease the total LDL circulating mass, can also shift the LDL profile towards less dense, larger species. This suggests that atorvastatin may also affect the atherogenic lipoprotein phenotype found in patients with combined hyperlipidemia.