Postulating an increased production of fibrin(ogen)olytic degradation products (FDP) and an abnormality of fibrinogen metabolism in polymyalgia rheumatica (PMR) and temporal arteritis (TA), we studied 16 PMR/TA patients and 10 control subjects using a sensitive radioimmunoassay for a specific type of FDP, namely, fibrin(ogen)-related D-antigen. Median serum D-antigen levels were increased five-fold in those 11 PMR/TA patients who were untreated compared with control subjects. In the five PMR/TA patients who were treated with prednisone the median D-antigen levels were not significantly different from those of the healthy controls. D-antigen concentration correlated significantly (r = 0.83) with ESR in the seven untreated PMR patients. In PMR patients prednisone therapy was followed by a reduction of serum D-antigen levels.