Cardiovascular disease is the leading cause of death in the world (WHO, 2009). One of many risk factors is low HDL cholesterol. HDLs are a heterogenous group of particles that are in dynamic flux in the circulation. Our understanding of the causes of low HDL and the metabolism of HDL is very poor. Using an isotope dilution protocol HDL kinetics will be studied in vivo in humans. A mathematical model will be developed and used to estimate the secretion and clearance of HDL subclasses and to identify the cause of gender differences. This will be the first step in a much larger collaboration which will take a systems biology approach to modelling cholesterol metabolism in vivo.