Objective Semapimod, a small molecule known to inhibit proinflammatory cytokine activity, was studied to determine the optimal dose necessary to achieve a response in patients with moderate to severe Crohn's disease (CD). Methods A randomised, double-blind, placebo-controlled trial (CD04) was carried out followed by an open-label extension study (CD05). The trial was conducted in international multicentre outpatient clinics and included patients with moderate to severe CD (Crohn's Disease Activity Index (CDAI) 250-400). Placebo was administered for 3 days; 60 mg semapimod intravenously for 1 day with placebo for 2 days; or 60 mg semapimod intravenously for 3 days. Participants who completed CD04 could participate in the open-label extension study, CD05, to receive up to five additional semapimod HCl 60 mg daily doses three times every 6-8 weeks. The main outcome measures were CDAI, Inflammatory Bowel Disease Questionnaire (IBDQ), Crohn's Disease Endoscopic Inflammation Score (CDEIS) and serum C-reactive protein (CRP) concentration. Results 152 patients were randomised in CD04. Responses for 1 and 3 day regimens were similar to placebo for CDAI (p=0.82), IBDQ (p=0.85), CDEIS (p=0.57) and CRP (p=0.40). The only noteworthy treatment-related safety finding was infusion reaction (phlebitis): 7.3, 34.8 and 62.7% for the placebo and 1 and 3 day semapimod treatment groups, respectively (p<0.001). In the open-label CD05 study (included 119 patients) a posthoc analysis showed that the mean CDAI improved in patients receiving 6 compared with <= 3 cumulative doses (204.1 +/- 83 vs 251.4 +/- 103.05, p=0.006). Conclusions Single and 3 day dosing of semapimod (<= 180 mg) was ineffective for the treatment of moderate to severe CD. However, cumulative dosing >= 360 mg was associated with decreased CDAI in a limited number of patients.