Traditionally, hyperlipidemias have been classified according to 6 phenotypes described by Fredrickson. These phenotypes are based on the electrophoretic patterns of lipoprotein level elevations that occur in patients with hyperlipoproteinemia. In recent years, the understanding of the genetic and biochemical basis of these disorders has revealed a large and diverse group of diseases, many of which have similar clinical expressions, exposing the limitations of the Fredrickson classification system. Despite the system's shortcomings, Fredrickson phenotypes are a useful tool for the discussion of these disorders. The understanding of the pathophysiology of these defects provides a basis for diagnosis and treatment.