We used in situ hybridization to search for the presence of viruses in synovial fluid cell preparations obtained from patients with various forms of knee arthropathies. One patient, presenting with acute reactive arthritis, was found to replicate parvovirus DNA in cells from synovial fluid whereas six other patients with various other forms of arthritis were negative for parvovirus infection. Five patients with osteoarthritis, constituting a control group in which an infectious etiology would not be expected, were consistently negative. In addition, no hybridization was found when synovial cell preparations of all patients were hybridized in situ with DNA probes specific for Epstein-Barr virus, cytomegalovirus, or enteroviruses.