Serum concentrations of C-Reactive protein (CRP) were measured in 40 patients with inflammatory bowel disease (22 with ulcerative colitis and 18 with Crohn's disease). In the evaluation of disease activity, CRP was a more sensitive (98.6% vs 71.4%) and specific (95% vs 84.6%) activity index in Crohn's disease than in ulcerative colitis. The CRP concentrations were significantly higher in Crohn's disease (57.7 +/- 55.7 mg/dl) than in ulcerative colitis (16.3 +/- 18.8 mg/dl), particularly in severely affected patients (104.8 +/- 54.3 vs 29 +/- 24.2 mg/dl). The CRP levels correlated significantly with clinical activity indices. There was poor or no correlation between CRP levels and the endoscopic activity indices, the extent of inflammatory bowel disease, and the days of hospitalization. We conclude that the determination of CRP levels provide a simple objective index of inflammatory activity with may be useful in the assessment, management and study of inflammatory bowel disease.