Prospective measurements were made of serum C-reactive protein levels and erythrocyte sedimentation rate in sixty-four patients with Crohn's disease and fifty with ulcerative colitis. The results were related to clinical assessment of disease activity. C-reactive protein levels were raised in both groups but were significantly higher in Crohn's disease than ulcerative colitis for all categories of disease severity: with mild disease the median and range of C-reactive protein concentration were 4, 0-65 mg/l in Crohn's disease v. 0, 0-15 mg/l in ulcerative colitis, P less than 0.01; in moderate disease the values were 15, 1-100 mg/l v. 3, 0-29 mg/l respectively, P less than 0.05 and in cases of severe disease, 85, 15-183 mg/l v. 12, 2-33 mg/l respectively, P less than 0.001. Erythrocyte sedimentation rate was also higher in Crohn's disease but did not closely reflect disease activity in individual patients. C-reactive protein levels corresponded closely with clinical and pathological indices of relapse, remission and response to therapy in patients with Crohn's disease. The precise assay of serum C-reactive protein provides an objective criterion of inflammatory activity, which may be useful in the assessment, management and study of Crohn's disease.