The prothrombin time (PT) and activated partial thromboplastin time (APTT) tests are often routinely ordered for hospitalized patients. Ordering patterns and clinical indications for the PT and APTT tests on the medical service at a teaching hospital were studied. Eighty-one percent of all patients admitted to the medical service had a PT and APTT test ordered. When compared with a modified version of guidelines for the use of the PT and APTT tests recently developed by the Medical Necessity Project of the Blue Cross and Blue Shield Associations of America and endorsed by the American College of Physicians, at least 70% of these tests were not clinically indicated. Many of the unindicated tests were ordered prior to invasive procedures or, apparently, out of habit. These inappropriate PT and APTT tests cost at least $60 948 per year for the medical service. Based on these findings, we suggest methods of reducing the inappropriate use of the PT and APTT tests.