Familial hypercholesterolemia (FH) is the most common genetic disorder associated with a high risk for premature atherosclerotic cardiovascular disease attributable to increased levels of LDL-cholesterol (LDL-C) from birth. FH is both underdiagnosed and undertreated. We describe the clinical, biological, and genetic characteristics of 147 patients in France with clinical FH (including a group of 26 subjects aged < 20 years); we explore how best to detect patients with monogenic FH. We retrospectively reviewed all available data on patients undergoing genetic tests for FH from 2009 to 2019. FH diagnoses were based on the Dutch Lipid Clinics Network (DLCN) scores of adults, and elevated LDL-C levels in subjects < 20 years of age. We evaluated LDLR, APOB, and PCSK9 status. The mutations of adults (in 25.6% of all adults) were associated with DLCN scores indicating "possible FH," "probable FH, and "definitive FH" at rates of 4%, 16%, and 53%, respectively. The areas under the ROC curves of the DLCN score and the maximum LDL-C level did not differ (p = 0.32). We found that the pediatric group evidenced more monogenic etiologies (77%, increasing to 91% when an elevated LDL-C level was combined with a family history of hypercholesterolemia and/or premature coronary artery disease). Diagnosis of monogenic FH may be optimized by screening children in terms of their LDL-C levels, associated with reverse-cascade screening of relatives when the children serve as index cases.< Réduire