This study is designed to provide efficacy and safety data for formoterol 10µg twice-a-day (b.i.d.) delivered by the Certihaler in patients with chronic obstructive pulmonary disease (COPD). This study is also designed to compare the efficacy and safety of therapy with formoterol (Certihaler)10µg b.i.d. added to tiotropium (HandiHaler) 18µg once daily (o.d.) compared with tiotropium (HandiHaler) 18µg o.d. monotherapy, and to compare the safety and efficacy of formoterol 10µg b.i.d. (Certihaler) with tiotropium 18µg o.d. (HandiHaler).