After a 1-week wash-out period, 6-minute walk test, and St. George's Respiratory Questionnaire (SGRQ) will then be administered prior to a run-in period of 1 week wherein all patients will be started on tiotropium alone. The patients will then be randomly allocated to either of the two arms for a two-week treatment, followed by one week washout, and crossed-over to the other treatment for another two weeks. The procaterol or placebo dose is given 2 hrs after the tiotropium dose.