BACKGROUND: Interleukin-11 is a mesenchymally derived cytokine with pleiotropic activities. A pilot study suggested therapeutic benefit of recombinant human interleukin-11 (rhIL-11) in patients with Crohn's disease. AIM: To determine the safety and preliminary estimate of efficacy of rhIL-11 in treating active Crohn's disease. METHODS: Patients with mild to moderately active Crohn's disease, defined as a Crohn's disease activity index (CDAI) > or = 220 and < or = 450, were enrolled in a multicentre trial. Stable doses of 5-aminosalicylates, antibiotics, 6-mercaptopurine or azathioprine were permitted with appropriate wash-in periods. Oral, intravenous or rectally administered corticosteroids were not allowed. Patients were randomized to 6 weeks of subcutaneous injection with rhIL-11 15 microg/kg or placebo weekly, or rhIL-11 7.5 microg/kg or placebo twice weekly. The primary end-point was per cent change in CDAI at week 6; the major secondary end-point was the proportion of patients in remission, defined as a 100 point decrease in CDAI and absolute CDAI < or = 150. RESULTS: Baseline characteristics were similar among the 148 evaluated patients (49 placebo, 49 rhIL-11 15 microg/kg once weekly, 50 rhIL-11 7.5 microg/kg twice weekly). Treatment was well-tolerated, with mild injection site reactions occurring more frequently among patients treated with rhIL-11. Headache, oedema, and increased platelet count occurred significantly more often in the rhIL-11 7.5 microg/kg twice weekly group, but not the 15 microg/kg once weekly group. There was a trend toward decreased mean per cent change in CDAI in the rhIL-11 15 micro/kg once weekly group vs. placebo (-31.5% vs. -18.5%, 95% confidence interval for the difference -27.9-1.6%). A significantly greater proportion of patients receiving rhIL-11 15 microg/kg once weekly achieved remission compared to placebo (36.7% vs. 16.3%, 95% confidence interval for the difference 3.4-37.4%; 16.4% for rhIL-11 7.5 microg/kg, N.S.). CONCLUSIONS: Weekly subcutaneous injection with rhIL-11 15 microg/kg is safe and effective in inducing remission in a subset of patients with active Crohn's disease.