The plasma immunoreactive concentration and the functional activity of C1 esterase inhibitor (C1INH) were measured in 17 samples from 15 patients with Crohn's disease (CD) and 10 samples from healthy volunteers. C1INH activity was measured by the chromogen substrate method and the immunoreactive concentration by the single radial immunodiffusion method. The functional activity was 95.7 +/- 4.6% in the controls. In CD it was 60.8 +/- 7.5% in the active stage (CDAI greater than 100) and 113.4 +/- 4.9% in the quiescent stage (CDAI less than or equal to 100). There were significant differences between the controls and both the active and quiescent stages (p less than 0.05). The activity was significantly lower in the active than in the quiescent stages (p less than 0.01). However, the difference in the immunoreactive concentration of C1INH in the active and quiescent stages was not significant; it was 27.8 +/- 3.5 mg/dl in the active stage and 33.7 +/- 2.0 mg/dl in the quiescent stage. This difference in the pattern of change between the immunoreactive concentration and the functional activity of C1INH might arise from the mode of C1INH action, with stoichiometric binding to substrates, giving rise to irreversible complexes. These results showed the functional consumption of C1INH in active CD, which may be an aggravating factor in the pathogenesis of the inflammatory process in the patient.