Trials of maintenance therapy in Crohn's disease are often underpowered, and there is need for objective markers of relapse. We assessed the relationship of whole gut lavage fluid cytokines to relapse in inactive Crohn's disease. Fifty-four patients with inactive Crohn's disease were prospectively assessed. Inactivity was determined as a Crohn's disease activity index of <150 and whole gut lavage fluid immunoglobulin G <10 microg/ml. All patients underwent whole gut lavage with analysis of IL-1beta and IL-8. Follow up was for one year. Patients with elevated whole gut lavage fluid IL-1beta (P < 0.004) and IL-8 (P < 0.02) had greater chance of relapse. Young age, short disease duration, and fistulating disease also relapsed more frequently. Multiple regression identified IL-1beta as an independent variable. In conclusion, an elevated whole gut lavage fluid IL-1beta in inactive Crohn's disease identifies patients at high risk of relapse.