Homozygous familial hypercholesterolemia (HoFH) is characterized by extremely elevated low-density lipoprotein-cholesterol (LDL-C) levels and early onset atherosclerotic cardiovascular disease despite treatment with conventional lipid-lowering treatment. This study was designed to assess LDL-C reduction with the proprotein convertase subtilisin/kexin type 9 inhibitor alirocumab in adult patients with HoFH. This randomized, double-blind, placebo-controlled, parallel-group, phase 3 study evaluated efficacy and safety of alirocumab 150 mg every 2 weeks. The primary endpoint was percent reduction from baseline in LDL-C versus placebo after 12 weeks of treatment. Patients (N = 69) were randomized 2:1 to alirocumab or placebo. At baseline, background lipid-lowering treatment included 67 patients receiving statin (59 patients on high-intensity statin); 50 patients on ezetimibe; 10 patients on lomitapide; and 10 patients undergoing apheresis. Mean baseline LDL-C was 259.6 mg/dl in the placebo group and 295.0 mg/dl in the alirocumab group. At week 12, the least squares mean difference in LDL-C percent change from baseline was -35.6% (alirocumab -26.9% vs. placebo 8.6%; p < 0.0001). Reductions (least squares mean difference) in other atherogenic lipids at week 12 were: apolipoprotein B, -29.8%; non-high-density lipoprotein cholesterol, -32.9%; total cholesterol, -26.5%; and lipoprotein(a), -28.4% (all p < 0.0001). No serious adverse events, permanent treatment discontinuations, or deaths due to treatment-emergent adverse events were reported during the double-blind treatment period. In the largest randomized controlled interventional trial in HoFH patients to date, alirocumab resulted in significant and clinically meaningful reductions in LDL-C at week 12. Alirocumab was generally well tolerated, with a safety profile comparable to that of placebo. (Study in Participants With Homozygous Familial Hypercholesterolemia HoFH ODYSSEY HoFH NCT03156621.).