Pentoxifylline has been reported to induce a decrease in plasma fibrinogen level in patients with arteritis. We report here ex vivo experiments in both healthy volunteers and patients with arteritis. In patients we have confirmed the decrease in plasma fibrinogen level during pentoxifylline therapy, and shown that the decrease is correlated with the improvement of the clinical symptoms, whereas no significant modification was noted in the healthy volunteers taking pentoxifylline. Therefore it is suggested that the decrease in fibrinogen induced in patients was due to an indirect mechanism, probably related to the pentoxifylline-induced decrease in TNF synthesis.