The performance of a rapid, fully automated HPLC assay for total homocysteine in plasma has made it possible to offer this analysis as a routine laboratory test. The reference interval in fasting individuals is 7-14 mumol/l. In EDTA-plasma, homocysteine remains stable for up to four days at room temperature. The most important indications for analysing homocysteine are deficiency of vitamin B12 or folate, premature cardiovascular disease and inherited homocystinuria. The clinical usefulness of the assay is based on the fact that the intracellular metabolism of homocysteine is dependent on vitamin B12, folate and vitamin B6. Moreover, homocysteine may be an atherogenic agent, and there is increasing evidence that a high level of homocysteine in plasma is an independent risk factor for developing premature atherosclerotic disease.