1. Forty-five patients with monarthritis of at least six months duration have been reviewed. Arthrotomy and synovial biopsy were carried out in every case. The period of follow-up varied from two to eight years from the time of biopsy. A re-examination of the biopsy material was made at the time of clinical assessment.

2. Twelve patients (27 per cent) were diagnosed as having definite or probable rheumatoid arthritis. Four patients (9 per cent) had psoriatic arthropathy and six (13 per cent) had a persistent monarthritis of undetermined type. A further four patients (9 per cent) had polyarthritis of undetermined type, ten (22 per cent) were diagnosed as having osteoarthritis and seven (16 per cent) were completely normal (self-limiting joint disease).

3. There was a moderately good relationship between biopsy findings suggestive of rheumatoid arthritis and the eventual clinical outcome.

4. Monarthritis of the wrist was followed in every case by the development of significant disease, either rheumatoid arthritis or psoriatic arthropathy.

5. Rheumatoid arthritis may remain monarticular for many years.

6. It is concluded that thorough investigation of patients with monarthritis is of considerable help in diagnosis and prognosis.