A study was conducted to evaluate compliance with the Sixth American College of Chest Physicians (ACCP) Consensus Conference on Antithrombotic Therapy guidelines for the prevention of venous thromboembolism (VTE) in hospitals. Using the HealthFacts database, hospitalized patients, age 40 years or more, with medical conditions at risk for VTE (surgical, trauma, and acute spinal injury patients) were identified. Hospital admissions on or after January 1, 2001, and concluded by March 31, 2005, were included if they met any of the following conditions as defined in the ACCP Consensus Conference on Antithrombotic Therapy guidelines: patients at risk of VTE medical conditions, major orthopedic surgery, general surgery, gynecological surgery, urologic surgery, trauma, neurosurgery, and acute spinal cord injury. Hospitalizations were identified using the International Classification of Diseases, 9th Revision, Clinical Modification codes. The primary objective was to examine whether patients received one of the indicated anticoagulants at the proper dosage and during the relevant hospital days as determined in the ACCP guidelines. Rates of compliance were assessed, and the reasons for guideline noncompliance were also determined. The overall compliance rate with ACCP guidelines was 13.3% (ranging from 2.8% for neurosurgery to 52.4% for orthopedic surgery) in the 123,304 hospital admissions that were reviewed. Only 15.3% of patients with at-risk medical conditions received prophylaxis in accordance with ACCP guidelines. Potential reasons for guideline noncompliance among selected conditions included the omission of prophylaxis, inadequate prophylaxis duration, and the wrong type of anticoagulant. A retrospective study found low rates of compliance with guidelines for thromboprophylaxis.