By means of the exploration of patients suffering from an active, unequivocal rheumatoid arthritis it was possible to show the unequivocal, and antiinflammatory effect of 4-phenyl-1,2-diphenyl-3,5-pyrazolidinedione (feprazone, Zepelin). Moreover, it was documented that thermography is a very suitable medium to furnish proof of this effect and can be used to the advantage of the patients in clinical routine. The physiological processes of the production and distribution of heat must be taken into account to avoid false interpretation of thermograms.