Very few investigators have studied the effect of contrast media on fibrinolysis. The results of those previous studies are contradictory and inconclusive. The purpose of this study was to evaluate the effect of ionic and nonionic contrast media on fibrinolysis in patients undergoing angiocardiography. Sixty-two patients randomly received either ionic contrast medium Hypaque-76 (n = 31) or nonionic contrast medium Ultravist-370 (n = 31). Plasma tissue plasminogen activator (t-PA), plasminogen activator inhibitor 1 (PAI-1), fibrinogen, and D-dimer were measured before and 20 minutes after the procedures. A significant increase of PAI-1 levels was seen in the Ultravist group but not in the Hypaque group. The t-PA and fibrinogen levels remained virtually unchanged in both groups. A significant increase of D-dimer concentrations was observed in the Hypaque group but not in the Ultravist group. The results of this study may in part explain the reason that the ionic contrast media produce fewer thromboembolic complications than the nonionic contrast media during the cardiac catheterization.