The author proposes that all major risk factors, including elevated serum low-density lipoprotein, cause atherosclerosis by increasing viscosity, creating larger areas of decreased blood flow, thereby perpetuating the interaction of atherogenic elements with the endothelium. Low-density lipoprotein causes increased viscosity by fostering erythrocyte aggregation. High-density lipoprotein protects against atherosclerosis by antagonizing erythrocyte aggregation, thereby decreasing viscosity. Implications of this theory are discussed.