A study to determine the utilization of calculated low density lipoprotein (c-LDL) cholesterol and measured low density lipoprotein (m-LDL) cholesterol was conducted. The test results of total cholesterol, triglyceride, HDL-cholesterol and m-LDL-cholesterol from the same individuals aged > or = 18 years who had the tests done at the Department of Clinical Pathology, Faculty of Medicine Siriraj Hospital during January to December 2004 were retrieved. The c-LDL-cholesterol level was computed using Friedewald formula. There were two data sets i.e. the m-LDL-cholesterol cut-off level derivation data set (784 subjects) and the m-LDL-cholesterol cut-off level validation data set (800 subjects). The study results revealed: 1) 2.6% of the subjects had blood triglyceride > 400 mg/dl hence c-LDL-cholesterol could not be computed, 2) the correlation between c-LDL-cholesterol levels and m-LDL-cholesterol levels from both data sets was very good (r > 0. 95, p 143, > 188, > 233 and > 254 mg/dl were highly correlated with c-LDL-cholesterol or = 100, > or = 130, > or = 160 and > or = 190 mg/dl respectively, 5) an application of m-LDL-cholesterol cut-off levels derived from the m-LDL-cholesterol cut-off level derivation data set to the m-LDL-cholesterol cut-off level validation data set showed that m-LDL-cholesterol 143, > 188, > 233 and > 254 mg/dl had accuracy in predicting c-LDL-cholesterol or = 100, > or = 130, > or = 160 and > or = 190 mg/dl of 100%, 99. 7%, 100%, 100% and 100% respectively, 6) the use of m-LDL-cholesterol levels as a guide for initiating lipid-lowering agents based on cut-off values of c-LDL-cholesterol levels led to an overuse of lipid-lowering agents in 3.6% to 42.9% of the patients and 7) Nomogram for transforming m-LDL-cholesterol to c-LDL-cholesterol was developed as well as a formula for transforming m-LDL-cholesterol to c-LDL-cholesterol (c-LDL-cholesterol = 0.89 x m-LDL-cholesterol). Therefore, m-LDL-cholesterol assay has a very limited use in managing individuals with suspected or known dyslipidemia. The use of m-LDL-cholesterol level as a guide for management of abnormal LDL-cholesterol conditions leads to an overuse of lipid lowering medications and an enormous expense of m-LDL-cholesterol assay.