A Cochrane review included 4 high quality studies with a total of 1 692 adult subjects with moderate to severe Crohn's disease. The subjects received 1 to 3 infusions of natalizumab (at a dosage of 300 mg or weight based dosages of 3, 4 or 6 mg/kg) or placebo. The studies lasted for 12 weeks. One infusion was significantly superior to placebo for induction of remission at 2 weeks (NNT=17) and for induction of clinical response at 4 weeks (NNT=8). Two infusions of natalizumab (3 to 4 mg/kg) were significantly superior to placebo for induction of remission at 6 weeks follow-up (NNT=10). Although 3 infusions (4 mg/kg) were significantly superior to placebo for induction of remission (NNT=12) and clinical response (NNT=10) at 12 weeks, these comparisons were not statistically significant at 10 weeks follow-up. Subgroup analysis demonstrated significantly greater clinical response and remission rates for natalizumab compared with placebo in patients with elevated CRP levels, active disease despite the use of immunosuppressants, or prior anti-tumor necrosis factor therapy. Natalizumab was generally well tolerated. Suspicion of rare serious adverse effects, like progressive multifocal leukoencephalopathy (PML), associated with the use of natalizumab together with certain immunosuppressants has been raised.