Proliferation and differentiation of vascular smooth muscle cells (VSMC) are central events in vascular pathobiology and play a major role in the development of stenotic and restenotic lesions. The proto-oncogene c-myc and other early cell cycle-regulating genes have been implicated in the induction of cell proliferation and differentiation under diverse pathophysiological conditions. In the present study we analyzed c-myc mRNA expression by indirect nonradioactive in situ hybridization technique (NISH) in human stenotic venous bypass grafts (n = 32) retrieved during re-do operations of coronary artery disease and compared the results with 28 native veins (vena saphena magna) from the same patients. Stenotic bypass grafts showed enhanced c-myc expression located predominantly in VSMC in the media and neointima (severity score: ++-+++, 32/32 stenotic veins). In native veins we observed only low levels of c-myc mRNA (severity score: +, 28/28 native veins), all signals were restricted to endothelial cells of either the innermost intimal layer or of the vasa vasorum. Our in situ hybridization studies demonstrate enhanced mRNA expression of the proto-oncogene c-myc in stenotic venous bypass grafts. These results suggest that--in analogy to other pathophysiological conditions--c-myc exerts essential regulatory functions in cellular events operative during the initiation and progression of venous bypass graft disease.