Comparison of the Crohn's Disease Activity Index (CDAI) with serum levels of the acute phase proteins during a 4-year study period of 50 patients with Crohn's disease has shown the serum C-reactive protein (CRP) to be a sensitive indicator of clinical status. Five patients are described in whom raised CRP levels reliably predicted relapse at a time when the patients were asymptomatic and the CDAI was normal. Measurement of serum CRP by radial immunodiffusion is cheap and simple, and if performed frequently in the follow-up of patients with Crohn's disease it may permit earlier, and therefore possibly more effective, treatment of relapse.