In a study of 25 giant cell arteritis patients, whose diagnoses were made by temporal artery biopsy, the authors compared the evolution of the erythrocyte sedimentation rate (ESR) with those of the acute phase proteins (APP): fibrinogen (F), C reactive protein (CRP), orosomucoid (O), haptoglobin (H) and alpha 2-globulins (alpha 2-G), before, during and after corticotherapy; 165 laboratory analyses were made. Prior to treatment, ESR was increased in 96% of the patients, O and H in 100%, F and CRP in 96% and alpha 2-G in 92%. CRP showed the greatest mean increase (21x). Statistically significant positive correlations were found between ESR and alpha 2-G, F, CRP and O. No significant relationship was observed between APP and the occurrence of ophthalmological complications or the length of treatment. The CRP level returned to normal within the first week of steroid therapy for 76% of the patients, before ESR, F and O. During the withdrawal phase of corticotherapy, an ESR greater than 30 mm almost always corresponded to an inflammatory syndrome and an ESR of less than 15 mm to its absence (kappa coefficient = 0.64, p less than 0.001); however, an ESR between 15 and 30 mm did not enable us to draw a conclusion as to the absence or presence of such a syndrome. After terminating steroid therapy, the relationship between ESR and an inflammatory syndrome was weaker (kappa coefficient = 0.57, p less than 0.02).(ABSTRACT TRUNCATED AT 250 WORDS)