Homocysteine is a nonessential, sulfur-containing amino acid involved in onecarbon (folate) metabolism. A number of inherited and acquired conditions cause increased accumulation of this metabolite in blood (homocysteinemia) and other biofluids. Homocysteinemia is a risk factor for cardiovascular disease, including recurrent thrombosis. Accurate measurement of total plasma homocysteine is an important element in the diagnostic evaluation of these disorders. While a number of different methods have been developed for this purpose, the focus of this unit will be on a specific technique utilizing liquid chromatography-tandem mass spectrometry, which provides several advantages in terms of speed, sensitivity, and specificity.