1. Using crossed immunoaffinity electrophoresis with free concanavalin A in the first dimension, we studied the glycan microheterogeneity of α 1 -antichymotrypsin in sera from patients with giant-cell arteritis and/or polymyalgia rheumatica, and its variation in the serum of several of these patients during induction of disease remission by prednisone therapy and possible further recurrence of giant-cell arteritis and/or polymyalgia rheumatica. 2. In the serum of patients with active disease we observed increased proportions of concanavalin A non-reactive and concanavalin A weakly reactive fractions. The results were expressed as the ratio of concanavalin A non-reactive fraction plus concanavalin A weakly reactive fraction to concanavalin A reactive fraction, called Rα 1 -ACT. An Rα 1 -ACT higher than 1.8 (upper normal value) was found in 30/31 sera from patients with active disease (sensitivity 97%) and in 2/22 sera from patients with inactive disease (specificity 91%). 3. The erythrocyte sedimentation rate and the serum C-reactive protein level, common markers of biological inflammation, are usually elevated in active giant-cell arteritis and/or polymyalgia rheumatica. The two parameters are commonly used to guide the therapy during the course of the disease, but they have no specificity. An erythrocyte sedimentation rate of greater than 30 mm/h was found in 30/31 sera from patients with active disease (sensitivity 97%) and in 5/22 sera from patients with inactive disease (specificity 77%). Our results show that α 1 -antichymotrypsin microheterogeneity provides a precise marker of giant-cell arteritis and/or polymyalgia rheumatica activity, particularly when the serum C-reactive protein level is normal or when the erythrocyte sedimentation rate is still high in treated patients. We found a better positive prediction value for Rα 1 -ACT than for erythrocyte sedimentation rate (94% and 86%, respectively). By determining the Rα 1 -ACT it is thus possible to follow the exacerbation of the disease when the dose of prednisone is tapered. 4. The correlation coefficients between clinical symptoms, erythrocyte sedimentation rate, levels of C-reactive protein or α 1 -antichymotrypsin, and Rα 1 -ACT values were determined. The highest correlation occurred between Rα 1 -ACT and erythrocyte sedimentation rate ( r = 0.57, P <0.0001). 5. Three case reports illustrated the value of determining Rα 1 -ACT as a useful adjunct test for the diagnosis of giant-cell arteritis and/or polymyalgia rheumatica.