This study assessed the effectiveness and safety of AMG145, a human monoclonal antibody to PCSK9, in patients with heterozygous familial hypercholesterolemia (HeFH) who were on intensive statin therapy but unable to reach LDL-C treatment goals. At 12 weeks, patients receiving 350 mg or 420 mg of AMG145 subcutaneously every 4 weeks saw LDL-C level reductions of 43% and 55% respectively, compared to a 1% increase in the placebo group. No serious adverse events occurred. AMG145 may provide an effective additional treatment to help more HeFH patients reach their LDL-C goals.