Cholesterol is a tightly regulated major structural component of the cell plasma membrane (PM) where it forms stoichiometric complexes with phospholipids and sphingolipids. The amount of cholesterol in the PM exhibits a regulatory role in basal activity of several biomolecular processes by direct binding to proteins and by indirect local environmental effects within the PM that are also coupled to overall cellular cholesterol homeostasis. The term "active cholesterol" refers to PM cholesterol not complexed to lipids, a cholesterol state that arises above a threshold mole fraction of cholesterol in the PM. Active cholesterol level in the PM provides a control mechanism for cellular cholesterol homeostasis through its recognition by membrane bound proteins that activate genes of cholesterol synthesis enzymes. Uptake of LDL, production and release of HDL as well as reversible storage of cholesterol in the cytosol by covalent modification are also regulated and dependent on PM cholesterol (thermodynamic) activity: active cholesterol. A number of human disease states have been found to have associated alterations in PM cholesterol and thus a method for its determination is described.