The transmembrane receptor Notch-1 regulates cell fate and differentiation and was suggested to identify a cell type with progenitor characteristics in newborn bovine articular cartilage. We show that Notch-1 is expressed on > 70% of BM-MSC in early passage monolayer culture. We also demonstrate that normal articular cartilage contains Notch-1+ cells and that the frequency is increased in OA. Most Notch-1+ cells in OA cartilage are located in the clusters of proliferating cells. These findings indicate that multipotential mesenchymal progenitor cells are present in articular cartilage from adult humans and that their frequency is increased in OA. This observation has implications for understanding the intrinsic repair capacity of articular cartilage and raises the possibility that these progenitor cells might be involved in the pathogenesis of arthritis.