The purpose of this study is to compare the safety and tolerability of levalbuterol HFA metered dose inhaler (MDI) versus racemic albuterol HFA MDI.

A randomized, modified-blind active-controlled multicenter, two-way crossover study of levalbuterol compared to racemic albuterol (using a spacer) in subjects 12 years of age and older with asthma. This study was previously posted by Sepracor Inc. In October 2009, Sepracor Inc. was acquired by Dainippon Sumitomo Pharma., and in October 2010, Sepracor Inc's name was changed to Sunovion Pharmaceuticals Inc.

Inclusion Criteria:

• Subject, male or female, must be at least 12 years of age at the time of consent.
• Female subjects 12-60 years of age inclusive must have a negative serum pregnancy test at study start.
• Women of child bearing potential must be using an acceptable method of birth control throughout the study.
• Subject must have a documented diagnosis of asthma for a minimum of 6 months prior to study start.
• Subject must be in good health with the exception of their reversible airways disease and not suffering from any chronic condition that might affect their respiratory function.
• Subject must have a chest X-ray or had one within 12 months prior to randomization.
• Subject must be able to complete the diary cards and medical event calendars reliably on a daily basis, understand dosing instructions and be able to demonstrate good MDI administration technique. Any minor subject who is not able to do this must have a parent/legal guardian who can assist them during the study with these activities.

Exclusion Criteria:

• Female subject who is pregnant or lactating.
• Subject who has participated in an investigational drug study within 30 days prior to study start, or who is currently participating in another clinical trial.
• Subject whose schedule prevents him or her from starting study visits before 9 AM.
• Subject who has travel commitments during the study that would interfere with trial measurements or compliance or both.
• Subject who has a history of hospitalization for asthma within 4 weeks prior to study start, or who is scheduled for in-patient hospitalization, including elective surgery during the course of the trial
• Subject with a known sensitivity to levalbuterol or racemic albuterol, or any of the excipients contained in any of these formulations.
• Subject using any prescription drug with which albuterol sulfate administration is contraindicated.
• Subject with currently diagnosed life-threatening asthma defined as a history of asthma episodes requiring intubation, associated with hypercapnia, respiratory arrest, or hypoxic seizures within 12 months prior to study start.
• Subject with a history of cancer (exception: basal cell carcinoma in remission).
• Subject with hyperthyroidism, diabetes, hypertension, cardiac diseases or seizure disorders.
• Subject with a history of substance abuse or drug abuse within 12 months preceding study start.
• Subject with greater than 10 pack year history of cigarette smoking or use of any tobacco products within 6 months of study start.
• Subject with a documented history of bronchopulmonary aspergillosis or any form of allergic alveolitis.
• Subject who has suffered from a clinically significant upper or lower respiratory tract infection in the 3 weeks prior to study start.
• Subject with unstable asthma; or who have had a change in asthma therapy; or a visit to the Emergency Department or hospital for worsening asthma within 4 weeks.
• Subject who is a staff member or relative of a staff member.