In patients with unstable angina pectoris, subjected (n = 20) or not subjected (n = 12) to percutaneous transluminal coronary angioplasty (PTCA), baseline superoxide anion (O.2-) generation by neutrophils in the coronary sinus blood was significantly higher than that found in the basilic vein blood of control healthy subjects (n = 12). During reperfusion following effective PTCA, neutrophil counts in the coronary sinus blood tended to decrease, an effect accompanied by a significant decrease in the neutrophil O.2- generation and enhancement of blood plasma lipid peroxidation as reflected by increased malonyldialdehyde concentrations.