Drug elution profiles must be studied in vitro to optimize a polymer-drug formulation during development of drug-eluting stents (DESs). Results from HPLC assays of drug contents and elution kinetics from a biodegradable sirolimus coating and a stable zotarolimus coating on coronary DESs are presented. Drug contents were assessed for crimped stents on the delivery system and expanded stents. The drug coating morphology and elution kinetics were demonstrated to be associated. Significant coating morphological defects were shown to cause deviations in the drug elution profile.