Serum C-reactive protein (CRP) concentration was determined by a radial immunodiffusion method in serum samples collected over a mean period of 19 months from a group of 17 patients with systemic lupus erythematosus (SLE). Thirty-two episodes of CRP elevation were detected. Twenty of these were associated with active SLE, and 9 with proven or possible infection or bone fracture. In 3 cases no explanation for CRP elevation was readily apparent. There were 8 instances of onset or exacerbation of lupus activity without accompanying CRP elevation. These data indicate that CRP elevation in the course of SLE is frequently associated with activation of lupus, and that detection of such elevation does not differentiate between lupus activity and infection.