The aim of this study was to evaluate the impact of concurrent nitroglycerin administration on the thrombolytic efficacy of recombinant tissue-type plasminogen activator (rTPA) in patients with acute anterior myocardial infarction (AMI). Sixty patients (53 men, 7 women; mean age 54 ± 7 years) with AMI entered the study. Thirty-three patients were randomized to receive rTPA alone (100 mg in 3 hours) (group A) and 27 to receive rTPA plus nitroglycerin (100 μg/min) (group B). Time from the onset of chest pain and delivery of rTPA was similar in the two groups of patients. Patients in group A had signs of reperfusion more often than the patients in group B (25 of 33 or 75.7% vs 15 of 27 or 55.5%, p < 0.05). Time to reperfusion was also shorter in group A than in group B (19.6 ± 9.4 minutes vs 37.8 ± 5.9 minutes, p < 0.05). Group B had a greater incidence of in-hospital adverse events (9 of 27 vs 5 of 33, p < 0.05) and a higher incidence of coronary artery reocclusion (8 of 15 or 53.3% vs 6 of 25 or 24%, p < 0.05). Peak plasma levels of rTPA antigen were higher in group A compared with group B (1427 ± 679 vs 512 ± 312 ng/ml, p < 0.01). In conclusion, concurrent nitroglycerin administration reduces the thrombolytic efficacy of rTPA in patients with AMI probably by lowering the plasma levels of rTPA antigen. The diminished efficacy of rTPA is associated with an adverse outcome.