The purpose of this study is to assess the Safety and Tolerability of REGN668 (how the body reacts to the drug) compared to placebo (an inert substance) in patients with moderate-to-severe extrinsic Atopic Dermatitis.

Inclusion Criteria:

• Clinical diagnosis of atopic dermatitis that has been present for at least 3 years before the screening visit
• Investigator's Global Assessment (IGA) score of >/= 3 at the screening and baseline visits
• >/= 15% body surface area (BSA) of AD involvement at the screening and baseline visits
• History of inadequate response to a stable (>/= 1 month) regimen of topical corticosteroids or calcineurin inhibitors as treatment for AD within 3 months before the screening visit
• Willing and able to comply with clinic visits and study-related procedures
• Patient able to read and understand, and willing to sign the informed consent form

Exclusion Criteria:

• A positive QuantiFERON® - TB (tuberculosis) Gold Test at the screening visit
• Known history of Human Immunodeficiency Virus (HIV), Hepatitis B or Hepatitis C and/or positive Hepatitis B surface antigen (HBsAg), positive Hepatitis C antibody (HCV)
• Treatment with an investigational drug within 8 weeks before the baseline visit
• Treatment with leukotriene inhibitors within 4 weeks before the baseline visit
• Treatment with systemic corticosteroids within 4 weeks before the baseline visit
• Treatment with topical corticosteroids, tacrolimus, and/or pimecrolimus within 1 week before the baseline visit
• Systemic treatment for AD with an immunosuppressive/immunomodulating substance within 4 weeks before the baseline visit
• Chronic or acute infection requiring treatment
• History of clinical parasite infection, other than treated trichomoniasis
• History of malignancy within 5 years before the baseline visit
• Any medical or psychiatric condition which, in the opinion of the investigator or the sponsor's medical monitor, would place the patient at risk, interfere with participation in the study, or interfere with the interpretation of study results
• Pregnant or breast-feeding women
• Unwilling to use adequate birth control, if of reproductive potential and sexually active