Synovial fibrosis was assessed in knee joints 2 weeks after intra-articular injection of Ad-transforming growth factor-β (TGF-β) combined with Ad-luciferase or Ad-Smad7. As a measure of fibrosis, the synovial width opposite the growth plates in the femur was measured (a) In addition, the amount of cells staining positive in immunohistochemically stained sections for procollagen type I was calculated with a computerized imaging system (b). The data are represented as an increase of the viral control. Histological representations of the measurements give an indication of actual thickness (c) and procollagen positive cells (d) There were no differences between viral and non-injected controls. TGF-β overexpression resulted in an increase in synovial thickness and number of procollagen type I-positive cells (p < 0.05). By co-expression with Smad7, most of the TGF-β-induced fibrosis was prevented (p < 0.05). Error bars display standard error.