Prothrombin time and partial thromboplastin time tests are commonly obtained prior to cardiac catheterization but the usefulness of the tests in these patients has not been examined. The records of 1,462 consecutive patients undergoing diagnostic catheterization were reviewed. All patients had received prothrombin time and partial thromboplastin time tests. The prevalence of abnormal test values was determined. The coagulation test values were correlated with history and physical evidence of bleeding risk, and also with bleeding complications. Abnormal coagulation values were found in 72/1,462, a prevalence of 4.9%. The history and physical exam predicted 63/72 (87.5%) of abnormal values, yielding a sensitivity of 87.5%, specificity of 100%, positive predictive value of 100%, and negative predictive value of 99.3%. The nine unpredicted abnormal values (0.6%, 9/1,462) were minimally abnormal and clinical decision making was not affected by the test result in any of the nine patients. There was no correlation between abnormal values and bleeding complications. Abnormal test values have a low prevalence in catheterization patients, 4.9% in this series, are highly predictable from history and physical exams, and in the absence of history or physical findings do not predict procedural bleeding complications. The tests should, therefore, be obtained only in a small percentage of patients, 4.3% in this series, who have an increased bleeding risk indicated by history or physical findings of anticoagulant use, excessive bleeding, liver disease, or malabsorption.