Inclusion Criteria:

• Male or female subjects aged 18 to 60 years at the time of the administration of investigational product.
• Written informed consent and HIPAA authorization (applies to covered entities in the US only) obtained from the subject/legal representative prior to performing any protocol-related procedures, including screening evaluations.
• Physician-diagnosed asthma with a duration of greater than or equal to 2 years by medical chart or subject report.
• Had an asthma exacerbation requiring urgent care in the year prior to screening.
• Meets NHLBI for persistent asthma in the 3 months prior to the current urgent healthcare visit.
• Current asthma exacerbation that must have lasted greater than or equal to 2 hours prior to arrival to the urgent healthcare setting.
• Requires at least 2 treatments of inhaled bronchodilators for the current asthma exacerbation in the urgent healthcare setting or within the emergency medical system (EMS)for greater than or equal to 1 hour.
• Shows an FEV1 or PEF of not more than 70% predicted after 1 hour of treatment of the current asthma exacerbation.
• Women of child-bearing potential, unless surgically sterile (including tubal ligation) and/or at least 2 years post-menopausal, must have used 2 effective methods of avoiding pregnancy (including oral, transdermal, or implanted contraceptives, intrauterine device, female condom with spermicide, diaphragm with spermicide, cervical cap, abstinence, use of a condom with spermicide by the sexual partner, or sterile sexual partner) from screening through the end of the study (Study Day 84; Cessation of birth control after this point should be discussed with a responsible physician).
• Men, unless surgically sterile, must likewise practice 2 effective methods of birth control (condom with spermicide or abstinence) and must use such precautions from Study Day 0 through Study Day 84.
• Otherwise healthy by medical history and physical examination.
• A chest x-ray that is normal for an asthmatic population and excludes alternative diagnosis per the investigation.
• Ability to complete the follow-up period until Study Day 168 as required by protocol.
• The investigator has determined that the subject is clinically stable and the FEV1, is greater than or equal to 30% predicted prior to receiving investigational product on Study Day 0.

Exclusion Criteria:

• Known history of allergy or reaction to any component of the investigational product formulation.
• Acute illness other than asthma at the start of the study.
• Fever > 38.6°C (>101.5°F).
• Current acute asthma attack is due to aspirin-induced asthma.
• Current asthma episode is an anaphylactoid/anaphylactic reaction presenting with acute bronchospasm.
• Evidence of clinically significant non-respiratory active infection, including ongoing chronic infection.
• History or current prolonged diarrhea, abdominal pain, and/or blood and mucus in stools or have minor symptoms AND have exposure to stream or lake water, been exposed to someone who has a parasitic infection (like a family member), or study subject has traveled outside the US and/or Canada within the last year.
• Use of immunosuppressive medication (except oral prednisone and inhaled and topical corticosteroids) within 30 days before randomization into the study.
• Have received Xolair(TM)within 6 months before randomization into the study.
• Receipt of immunoglobulin or blood products within 30 days before randomization into the study.
• Receipt of any investigational drug therapy within 6 months before the first dose of investigational product in this study through Study Day 168.
• History of primary immunodeficiency.
• Previous medical history, or evidence, of an intercurrent illness that may compromise the safety of the subject in the study.
• History of clinically significant abnormality on ECG in the opinion of the investigator.
• Pregnancy (must have a negative serum pregnancy test prior to the first dose of investigational product).
• Breastfeeding or lactating woman.
• History of treatment for alcohol or drug abuse within the past year.
• Diagnosis of COPD by a healthcare professional.
• Evidence of any clinically significant systemic disease on physical examination.
• History of cancer except basal cell carcinoma or in situ carcinoma of the cervix treated with apparent success with curative therapy > 1 year prior to entry or other malignancies treated with apparent success with curative therapy > 5 years prior to entry.
• Known exposure to inhaled occupational agents or fumes with an established diagnosis of occupational asthma.
• Any condition (ie, impending ventilatory failure or hemodynamic compromise) that,in the opinion of the investigator, would interfere with evaluation of the investigational product or interpretation of study results.
• Any employee of the clinical study site who is involved with the conduct of the study.
• History of cigarette smoking > 20 pack years.
• Previously received MEDI-563.
• Asthma exacerbation due to acute inhalational exposure.