In rheumatoid arthritis (RA) the synovial fluid lymphocyte count closely parallels the synovial fluid dialysable hydroxyproline, a marker of collagen resorption. This observation stresses the primordial role of lymphocytes in RA joint injury. The actual and eventually potential destruction of any single joint, as expressed by synovial fluid dialysable hydroxyproline levels, seems to reflect the general picture of disease activity as evaluated by the number of active joints (joint count), ESR and rheumatoid factor titres (latex fixation). Although urinary hydroxyproline levels are generally within the normal range in RA, they can be used as index of articular tissue destruction and as a parameter of overall disease activity when groups of patients are studied longitudinally in detail. The present study sheds no further light on the significance of synovial fluid and blood non-dialysable hydroxyproline levels.