For many years evidence for a viral aetiology of connective tissue diseases such as systemic lupus erythematosus and rheumatoid arthritis has been sought by a variety of methods, including attempts at direct isolation, the study of rheumatoid synovial cells, ultrastructural examination of pathological material and assays of anti-viral antibody. No convincing proof has yet been obtained. However, a better understanding of the mechanisms of viral persistence, and the ways in which host defences can be subverted by viral infections has prompted other ways of approaching this problem. A viral aetiology for this group of diseases remains an attractive but unsubstantiated hypothesis.