We measured the serum C reactive protein (CRP) levels by a sensitive immunochemistry method in 101 outpatients with Crohn's disease enrolled in a prospective longitudinal study. At entry, 51 of the 101 patients had raised CRP levels, which showed good correlation with the clinical score. However, one-third of the patients with clinically active disease had normal CRP while one-third of the patients in clinical remission had a raised CRP. The longitudinal study showed that (a) in patients with active disease, CRP levels tended to decrease as the disease went into remission; (b) in patients who achieved remission, the likelihood of clinical relapse after 2 years of entry was higher in those with persistently raised CRP than in those with persistently normal CRP; and (c) in patients with quiescent disease at entry who remained in remission throughout the first year of the study, the likelihood of clinical relapse during the second year of the study was higher for those with persistently raised CRP than for the others. This study confirms that serum CRP is a useful laboratory index for supplementing clinical scores in patients with Crohn's disease, in monitoring the response to treatment, in helping to predict the course of the disease, and in contributing to defining subgroups of patients.