Anticoagulation in hospitalized patients is frequently associated with medication errors. To determine the extent and severity of this problem, we reviewed consecutively reported, anticoagulation-related medication errors over a 3-year period. We identified 130 medication errors. There were 1.67 medication errors for every 1,000 patients treated with anticoagulants. These were most often associated with unfractionated heparin (66.2%), followed by warfarin (21.5%), low-molecular-weight heparin (9.2%), argatroban (1.5%), and lepirudin (1.5%). There were no deaths attributed to any anticoagulant medication errors, but 6.2% of patients required medical intervention and 1.5% needed prolonged hospitalization. Medication errors frequently occur with anticoagulant therapy in hospitalized patients.