In a prospective study over 2 years, serum C-reactive protein (CRP) concentration and erythrocyte sedimentation rate were measured serially in thirty-eight patients with various types of necrotizing systemic vasculitis. The CRP concentration was always elevated in patients with active vasculitis 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. In contrast the sedimentation rate responded more slowly to changes in disease activity and did not necessarily reflect the level of inflammation at a particular time. These results, together with the commercial availability of rapid and precise assays for CRP, indicate that serial measurement of the serum CRP fills the urgent need for an objective index of the activity of the systemic vasculitides and their response to therapy.