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Summary Objective : To accurately localize type-1 plasminogen activator inhibitor (PAI-1) both in atherosclerotic and normal human coronary arteries and to determine which cells are responsible for its production. Design : Immunohistochemistry and in situ hybridization over paraffin embedded samples obtained from patients subjected to heart transplant in ‘La Fe’ University Hospital, Valencia, Spain. Results : PAI-1 mRNA is clearly detectable in atherosclerotic coronary artery, both in endothelial cells and in smooth muscle cells (SMCs) in the connective tissue of the thickened intima, while in the core of the plaque macrophages are the main PAI-1 producer cells. This pattern is coincident with PAI-1 antigen localization. In arteries appearing to be normal, PAI-1 antigen and mRNA were only slightly detectable or not at all. Conclusion : PAI-1 expression and antigen are increased in endothelial cells from atherosclerotic arteries. PAI-1 mRNA and antigen are also present in the SMCs from thickened arteries and in macrophages in the core of the plaque. These results suggest that PAI-1 may be involved in the development, evolution and final outcome of the atherosclerotic plaque.