C-reactive protein (CRP) is an acute-phase protein that is produced in large amounts by hepatocytes, upon stimulation by the cytokines interleukin-6, tumor-necrosis-factor-alpha and interleukin-1beta, during an acute-phase response. CRP is an objective marker of inflammation and, in gastrointestinal diseases such as Crohn's disease and acute pancreatitis, its levels correlate well with clinical disease activity. In contrast to its use as a marker in Crohn's disease, however, CRP is a less reliable marker of inflammation and disease activity in patients with ulcerative colitis, except perhaps for severe, extensive colitis. The increased production of CRP after an acute-phase stimulus, such as active gut inflammation, might explain why strong anti-inflammatory agents, such as anti-tumor-necrosis-factor-alpha antibodies and other biologic agents, work particularly well in patients with increased levels of CRP. CRP is also useful as a laboratory marker to predict prognosis and relapse in patients with Crohn's disease and acute pancreatitis. Elevated CRP levels have been associated with an increased risk of colorectal cancer and are a marker of poor prognosis, indicating more advanced disease and, possibly, reduced survival. An important question that remains is how often CRP levels should be measured. Until there are more data, the use of CRP and of other biomarkers should be seen as an additional tool that aids clinical observation and physical examination, but that cannot replace it.