Lymphocytes from the site of disease suggest adenovirus is one cause of persistent or recurrent inflammatory arthritis. The assessment of synovial lymphocyte reactivity to adenovirus antigen stimulation was undertaken in patients with persistent or recurrent inflammatory arthritis. The 3H-thymidine uptake procedure was employed, incorporating multiple microbiological antigens. Five patients were found with repeated maximal responses to adenovirus antigen; in one of these adenovirus nucleotide sequences were present in a synovial biopsy specimen. It is concluded that adenovirus may be one cause of persistent or recurrent inflammatory arthritis.