Serum levels of soluble interleukin 2 receptors (sIL-2R) were measured in 21 patients with polymyalgia rheumatica (PMR)/giant cell arteritis (GCA) prior to steroid treatment. These levels were significantly elevated in patients with PMR/GCA compared with healthy controls (p = 0.002). A significantly longer duration of morning stiffness (p = 0.005) was observed in patients with a high concentration of sIL-2R. A significant correlation was observed at diagnosis between sIL-2R and erythrocyte sedimentation rate (ESR) (p = 0.01) and between ESR and C-reactive protein (CRP) (p = 0.005). We investigated prospectively a group of 10 patients over a period of 6 months of prednisone therapy. At the end of the study sIL-2R levels fell significantly compared to pretreatment values (p = 0.02), but remained significantly higher compared to controls (p = 0.02). ESR and CRP values also fell significantly compared to pretreatment levels (p = 0.0001 in both cases). We observed a significant correlation between the decrease in ESR values and the decrease in sIL-2R and CRP levels after 6 weeks (p = 0.01 in both cases) and after 6 months of therapy (p = 0.002 and p = 0.05). sIL-2R may be considered a useful serologic marker for monitoring response to steroid therapy in patients with PMR/GCA. This laboratory variable correlated more closely with ESR than with CRP. The presence of elevated levels of sIL-2R is likely to reflect T cell activation occurring in PMR/GCA. T lymphocyte activation persisted after 6 months of steroid therapy, despite rapid and continuous control of disease manifestations.