The efficiency of screening for dyslipoproteinemias associated with hyperlipoproteinemia or with hypolipoproteinemia was examined in 8449 white examinees from the Lipid Research Clinics Prevalence Study. A two-stage lipid screening approach was used. A positive screening test for hyperlipidemia was defined as an elevated level of plasma total cholesterol or triglyceride at both visit 1 and visit 2. A positive screening test for hypolipidemia was defined as a reduced plasma total cholesterol level at both visits. The syndromes of dyslipoproteinemia were defined according to the lipoprotein pattern present at visit 2. When hyperlipidemia was used as a screening tool, generally only about one-half or less of the cases of hyperlipoproteinemia were detected (high proportion of false-negative results). Of all the hyperlipidemic participants, more than 80% had hyperlipoproteinemias (type I, IIa, IIb, III, IV, or V), and such lipid screening therefore provided a lower proportion of false-positive than false-negative results. Most of the participants (greater than 98%) without hyperlipoproteinemia were correctly identified (true negatives). The efficiency of screening for hypolipoproteinemia was in general poorer than that found for hyperlipoproteinemia. Our results were found to be related, in part, to the marked regression to the mean of plasma cholesterol and triglycerides at the extremes of the distributions and to the wide variety of dyslipoproteinemias that can be present within a given range of plasma cholesterol or triglyceride values. The results emphasize the importance of measuring plasma lipoproteins in the patient who is being evaluated for dyslipoproteinemia.