Key Points Question What is the safety and tolerability of zerlasiran and its effect on lipoprotein(a) serum concentrations? Findings The trial enrolled 32 healthy participants and 36 patients with cardiovascular disease and lipoprotein(a) concentrations 150 nmol/L or greater. No serious adverse event occurred. The median change from baseline in lipoprotein(a) concentration 365 days after single doses for placebo, 300 mg, and 600 mg were 14%, −30%, and −29%.