By using National Cholesterol Education Program guidelines for serum cholesterol (less than 200 mg/dl is designated "desirable," and 200 to 239 mg/dl is designated "borderline-high," and greater than or equal to 240 mg/dl is designated "high"), low-density and high-density lipoprotein (LDL, HDL) cholesterol levels and triglyceride levels were quantitated in 897 self-referred fasting subjects to assess the potential for coronary risk misclassification. With cholesterol less than 200 mg/dl, misclassification was arbitrarily identified by an LDL level greater than or equal to the 75th percentile, a triglyceride level greater than or equal to the 90th percentile, or an HDL level less than or equal to the 10th percentile. With the cholesterol level in the 200 to 239 mg/dl range, misclassification was identified by an LDL level greater than or equal to the 75th percentile, a triglyceride level greater than or equal to the 90th percentile, and an HDL level less than or equal to the 10th percentile or greater than or equal to the 90th percentile (or both). With a cholesterol level greater than or equal to 240 mg/dl, misclassification was identified by an HDL level less than or equal to the 10th percentile, or greater than or equal to the 90th percentile. With the cholesterol level less than 200 mg/dl, misclassification is rare, occurring in 14.5% of the subjects. With the cholesterol level in the 200 to 239 mg/dl range, and greater than or equal to 240 mg/dl, misclassification occurred in 46.7% and 17.6% of the subjects, respectively. The importance of routine lipoprotein analysis when the cholesterol level is greater than or equal to 240 mg/dl is emphasized by the finding that 65% of the subjects in this category had top quartile LDL levels, 8% had bottom decile HDL levels, and 30% had top decile triglyceride levels. To avoid misclassification, fasting HDL, LDL, and triglyceride levels should probably be measured in all subjects with screening cholesterol levels greater than or equal to 200. There is remarkably little misclassification with top quartile LDL or bottom decile HDL levels (or both) when the cholesterol level is less than 200 mg/dl.