Pulmonary embolism is a rare but life-threatening complication of cardiac catheterization. Underlying deep venous thrombosis (DVT) is often not detectable clinically. To determine the true incidence of DVT the authors prospectively studied 450 consecutive patients (29% women, 71% men, mean age: fifty-eight years) undergoing a diagnostic cardiac catheterization. Patients were examined clinically and by duplex sonography with a high-resolution (5 or 7.5 MHz) transducer before and twenty-four hours after catheterization before mobilization. Duplex sonography excluded complete proximal DVT in all patients. Only partial occluding thrombi (pDVT) were detected in 11 (2.4%) patients. The thrombi were always localized at the puncture site. In 2 patients a difference was found in the circumferences of the legs, but no other clinical signs of DVT were seen. With use of continuous wave (cw) Doppler sonography, only 3 of these 11 patients (27%) showed a spontaneous (s) sound. Phlebography was performed in 4/11 patients (36%). In 2 patients the diagnosis was confirmed; in 1 patient extravenous compression was assumed, and the other demonstrated a normal-appearing phlebography at the time of investigation. Logistic regression analysis yielded a 3.5 times higher risk for developing a pDVT if a venous puncture was performed in addition to arterial puncture. Furthermore a 9.8 times higher risk was found if more than one venous puncture was necessary. During the follow-up no patient developed clinical signs of pulmonary embolism. The results of this study demonstrate that DVT is a rare complication of cardiac catheterization (0/450 patients), but pDVT occurred in 2.4%. Risk factors for pDVT are the venous puncture itself and multiple puncture attempts. Clinical relevance of pDVT remains to be determined.