OBJECTIVE To assess compliance with the 2001 consensus guidelines of the American College of Chest Physicians (ACCP) regarding administration of vitamin K1 to reverse the anticoagulant effect of warfarin. METHODS Retrospective chart review. METHODS University teaching hospital. METHODS Fifty-five adult inpatients who received both warfarin and vitamin K1 between September 2001 and January 2002. RESULTS The patients' medical records were evaluated; data were collected on patient demographics and on vitamin K1 dosage and route of administration, warfarin dosage, and international normalized ratio (INR) before and after vitamin K1 administration. Administration routes and 87 doses of vitamin K1 prescribed for the 55 patients were assessed for compliance with the ACCP guidelines. Administration routes were subcutaneous (40.2% of doses), intravenous (35.6%), oral (13.8%), and intramuscular (10.3%). The most frequently prescribed dose of vitamin K1 was 10 mg (32.2%), followed by 2 mg (21.8%) and 5 mg (18.4%). Rates of compliance with the ACCP guidelines categorized by INR value were as follows: INR below 5, 12.2%; INR 5-9, 27.8%; INR between 9 and 20, 26.7%; and INR above 20, 0%. Four patients had documented episodes of bleeding and received seven doses of vitamin K1. Twenty-six patients received fresh frozen plasma with vitamin K1. Overall compliance with ACCP-recommended doses and routes of vitamin K1 was only 17.2%. CONCLUSIONS The most frequently prescribed administration routes for vitamin K1 were subcutaneous and intravenous, indicating that the oral route is often not used as recommended. The vitamin K1 doses prescribed for reversal of warfarin anticoagulation were highly variable, and for most (83%) patients, the recommended guidelines were not followed. The clinical significance of noncompliance with the ACCP guidelines for vitamin K1 administration warrants further study.