The distribution of fibronectin, an extracellular matrix glycoprotein which plays a part in fibrosis and tissue repair, has previously been described using immunohistochemical methods. These do not differentiate between locally synthesised and plasma derived fibronectin. In this work the distribution of cells actively synthesising fibronectin was assessed by in situ hybridisation using a radiolabelled antisense RNA probe in synovial biopsy samples from patients with rheumatoid arthritis, osteoarthritis, ankylosing spondylitis, and control subjects without inflammatory disease. Large amounts of fibronectin mRNA were found specifically in synovial lining cells, providing evidence for the local production of fibronectin in the synovium. Levels of fibronectin mRNA were variable between patients. These differences were not related to the diagnosis or to the subintimal inflammatory cell infiltrate; where there was synovial lining cell hyperplasia there was a concomitant increase in the number of cells containing fibronectin mRNA, which was consistent with increased levels of immunoreactive fibronectin at this site. Increased levels of fibronectin in synovial fluid in patients with rheumatoid arthritis may be due to an increased number of lining cells secreting the protein, rather than upregulation of the gene by these cells.