Deep vein thrombosis and pulmonary embolism (DVT/PE) are common complications after surgery and are associated with substantial excess mortality and length of stay. International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) codes recorded in hospital claims have been used to identify and study DVT/PE, but the validity of this method is not well studied. Identification of postoperative DVT/PE events were compared using ICD-9-CM codes and medical record abstraction in random samples of hospital discharges of Medicare beneficiaries in 2002-2004. Among 20,868 eligible surgical hospitalizations, 232 DVT cases and 95 PE cases were identified by ICD-9-CM codes; 108 DVT cases and 31 PE cases by medical record abstraction; 72 DVT cases and 23 PE cases by both methods. The resulting estimates of PPV of ICD9-CM coding were 31% (72/232 cases) for DVT, 24% (23/95) for PE, and 29% (90/308) for DVT/PE combined. The resulting sensitivity estimates were 67% (72/108 cases) for DVT, 74% (23/31) for PE, and 68% (90/133) for DVT/PE combined. ICD-9-CM codes in Medicare claims are sensitive but have limited predictive validity in identifying postoperative DVT/PE. Improvements in the validity are needed before the indicator can be used for safety performance assessment.