Retroperitoneal hematoma (RPH) following cardiac catheterization is an infrequent (0.15% incidence) but morbid complication. During a 13-month study period, 11 patients with a significant RPH requiring operative intervention were identified. The mean transfusion requirement was 8.7 units, with two deaths as a consequence of their RPH. Adjunctive cardiac procedures included percutaneous transluminal coronary angioplasty (five), stent placement (one), and thrombolysis (two). Two patients had RPH following aortography. Suspicion of RPH was most frequently prompted by a falling hematocrit (73%), with hypovolemic shock (systolic blood pressure < 90) in 64%. Lower quadrant or flank pain occurred in four patients. Lower extremity pain occurred in five patients due to femoral nerve compression. Of six patients with a preoperative femoral nerve palsy, complete resolution occurred in four cases. RPH following femoral arterial puncture is a cause of significant morbidity, particularly in the anticoagulated patient. Postcatheterization anticoagulation and high arterial puncture were the principal risk factors (p < 0.001). Early recognition is essential and should be prompted by a falling hematocrit, lower abdominal pain, or neurological changes in the lower extremity. There should be a low threshold for performing abdominopelvic CT scans in such patients. Management of RPH must be individualized: 1) patients with neurological deficits in the ipsilateral extremity require urgent decompression of the hematoma, 2) anticoagulation should be stopped or minimized, 3) hematoma progression by serial CT necessitates surgical evacuation and repair of the arterial puncture site.