OBJECTIVE Efalizumab is a monoclonal antibody targeting CD11a, an adhesion molecule involved in the activation and trafficking of T-lymphocytes. This agent has proven efficacy in the treatment of psoriasis. We performed an open-label study to evaluate the efficacy and safety of efalizumab in Crohn's disease (CD). METHODS Fifteen subjects with moderate to severe CD (Crohn's Disease Activity Index [CDAI] score 220-450) and who were refractory or intolerant to standard therapy, received a weekly 1 mg/kg subcutaneous injection of efalizumab for 8 weeks. The primary endpoint was clinical response (decrease in the CDAI score of at least 70 points) at week 8. Secondary endpoints included change in mean CDAI scores, the proportion of subjects who achieved clinical remission (CDAI score ≤ 150), change in the Inflammatory Bowel Disease Questionnaire (IBDQ) scores, and report of adverse events. RESULTS At 8 weeks, ten (67%) subjects had clinical response and six (40%) were in remission. The mean baseline and week 8 CDAI scores were 300 and 167 respectively (P < 0.001). Mean IBDQ scores at baseline and week 8 were 124 and 168 respectively (P < 0.001). One subject with Crohn's colitis had pre- and post-treatment colonoscopy that demonstrated mucosal healing. No serious adverse events occurred. CONCLUSIONS Efalizumab induced a clinical response in the majority of subjects with moderate to severe CD in this small, open-label pilot study. There were no serious adverse events reported during this short-term trial.