Serial measurements of the serum concentration of C-reactive protein were made in 38 patients with Wegener's granulomatosis during a period of 6 years. The concentration was always elevated when the disease was active, even in patients receiving immunosuppressive treatment, and fell rapidly in association with clinical remission induced by immunosuppression. During periods of complete remission, in the absence of any intercurrent condition, the value remained within the normal range. The correlation between C-reactive protein level and disease activity was much closer than that between erythrocyte sedimentation rate and disease activity. These results indicate that serial measurement of the serum C-reactive protein fills the urgent need for an objective index of the activity of Wegener's granulomatosis and its response to therapy.