Psoriatic arthritis (PA) may respond to disease-modifying antirheumatic therapy. The value of assessing disease activity with indices devised for rheumatoid arthritis (RA) was investigated in 72 patients with seronegative PA. Thirty patients had a peripheral polyarthritis including the distal interphalangeal joints (DIPJs) and 15 a symmetrical arthritis sparing DIPJs (RA-like). Significant correlations (Spearman rank test) were seen between the clinical variables (pain score, grip strength, Ritchie articular index and a summated index of disease activity) in these two groups. Ten patients with a markedly asymmetrical arthritis showed a poor correlation between clinical variables. Although the objective indices - erythrocyte sedimentation rate (ESR) and C-reactive protein - correlated together in the first two groups, the ESR correlated solely with clinical indices, and then only in RA-like patients. These results cast some doubt on the value of assessment methods based on RA when evaluating subgroups of PA other than RA-like disease.