Interstitial cells in the scars of human myocardial infarctions of different postinfarction times (6 hours to 17 years old) were characterized by antibodies to alpha-smooth muscle actin (ASMA), vimentin, and desmin. Basal lamina deposition was studied with antibodies to the basal lamina protein type IV collagen. Nonvascular spindle-shaped cells expressing ASMA were present within 4 to 6 days after infarction. These cells co-expressed vimentin but no desmin and showed discontinuous basal lamina deposition. In electron microscopy these cells showed features characteristic of myofibroblasts. The spindle-shaped cells persisted for a long period of time and could even be identified 17 years postinfarction. In transmural infarctions they were orientated parallel to the endocardium and epicardium. In nontransmural patchy infarctions they showed an orientation adjacent to the cardiomyocytes and appeared to be less dense than in the transmural infarctions. In conclusion, myofibroblasts expressing ASMA persist within human myocardial scars and show a preferential alignment that may be the result of the continuous mechanical stress caused by the ongoing contraction and relaxation of the surrounding viable myocardium.