In patients with acute myocardial infarction, a persistently occluded infarct related coronary artery, despite a correct thrombolysis, is associated with an unfavourable prognosis. Therefore, identification of variables predictive of ineffective thrombolysis is crucial to identify patients at higher risk of thombolysis failure. To investigate whether or not acquired or congenital thrombophilic factors had a role in the ineffective thrombolysis we designed this study in which patients treated with intravenous thrombolysis for a ST segment elevation myocardial infarction were blind tested for the thrombophilic factors on the occasion of the coronary angiography performed within 30 days from the thrombolytic treatment. Patients with known factors influencing metabolism and circulating levels of homocysteine were excluded from this study. From October 2003 to May 2004, 104 consecutive patients treated with intravenous thrombolysis for a ST segment elevation myocardial infarction were available for this study, 3of these refused to participate in the study All patients underwent,within 30 days from thrombolysis, a coronary angiography and of the 101 participating in the study, 40 resulted occluded while 61 had a patent artery. In these 101 patients we blind tested the levels of ATIII,PC,PS; moreover, we determined also the levels of homocysteine, the presence of Lupus Anticoagulant (by mean of DRVVT and Silica Clotting Time) and ACA of IgG type as well as the Plasminogen levels. Furthermore, blood samples were also analysed by PCR technique for the presence of Factor V Leiden, the G20210A Factor II mutation and the C677T mutation in the MTHFR gene. Surprisingly, patients with MTHFR 677TT homozygosis had a significantly higher prevalence of occluded infarct artery (73%) vs those with MTHFR 677CT/CC genotype (30%, P=0.0008); frequency of MTHFR 677TT homozygosis was 4-fold higher in patients with occluded vs those with a patent vessel (40% vs 10%, P=0.0008). MTHFR 677TT genotype predicted the risk of failed thrombolysis with a specificity of 90% and multivariate analysis showed that MTHFR 677TT homozygosis was independently associated with an occluded artery (odds ratio 3.8, 95% confidence interval 1.1–9.1; P=0.03). None of the other studied factors at multivariate analysis influenced the thrombolysis failure. Moreover, patients with occluded infarct vessel and MTHFR 677TT genotype had the highest homocysteine levels (P=0.011). Our findings indicate that in patients with acute myocardial infarction, MTHFR 677TT homozygous is independently associated with a persistently occluded infarct-related artery after thrombolysis.