We investigated the area of the tidemark in osteoarthritic and normal cartilage by stereology as well as the number of vessels penetrating the tidemark. The area of the tidemark was significantly increased in osteoarthritis, as was the number of vessels penetrating the tidemark. These three-dimensional findings are new compared to previous two-dimensional findings where it has been discussed whether there were any differences between osteoarthritic cartilage and normal cartilage with respect to duplications of the tidemark and vascularisation. The present findings using stereology may erase the uncertainty of previous findings, and support the hypothesis that primary osteoarthritis is partly of vascular origin.