This study is a double-blind, randomized, active-controlled, multicenter, parallel-group trial of levalbuterol in adult subjects with acute reversible airways disease. Approximately 600 subjects will be enrolled and study participation will consist of 2 periods: Period I (Acute Period): Double-blind treatment in the Emergency Department (ED) or Clinic until disposition, for a maximum of 24 hours of double-blind treatment and Period II (Post-Acute Period): Subjects discharged from the ED or Clinic will continue double blind treatment (with the same treatment provided as rescue medication as MDI) for approximately 10 days. Subjects will be contacted by telephone 30 days post discharge to assess relapse. 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.