We have evaluated whether low density lipoprotein (LDL) receptor activity of stimulated lymphocytes, as measured by an improved flow cytometric assay, may be used to diagnose familial hypercholesterolemia (FH). Cells were isolated from 75 children suspected from strict clinical criteria to be FH heterozygotes and from 29 normal children. DNA from the FH patients were also subjected to molecular genetic analysis of the LDL receptor gene in order to confirm the clinical diagnosis. A molecular genetic diagnosis of FH was obtained in 68 of the 75 patients; 67 of these had a low (below 70% of normal) receptor activity and 1 had a borderline (71%) activity. By contrast, 28 of the normal children showed a normal (above 80%) and 1 a borderline (78%) receptor activity. Of the 7 patients in whom no mutation in the LDL receptor gene was found, 4 showed a normal, 1 a borderline, and 2 showed a low activity. In summary, measurement of LDL receptor activity allowed us to separate between genetically diagnosed FH heterozygotes and healthy children. The combined use of LDL receptor activity measurements and molecular genetic analysis allows us both to diagnose and exclude FH in children suspected to suffer from this disease.