If subjects meet all entry criteria at the end of the tamsulosin hydrochloride run-in period, subjects will be randomized to 1 of 2 treatment groups (mirabegron or placebo) for 12 weeks of treatment in addition to the continuation of tamsulosin hydrochloride 0.4 mg QD. Those subjects randomized to mirabegron will start at 25 mg and will increase to 50 mg after 4 weeks. Those subjects randomized to placebo will start blinded product matched to the mirabegron 25 mg tablet and will increase to blinded product matched to 50 mirabegron after 4 weeks. Once a subject has increased dose, the subject will remain on that dose for the remainder of the study unless for safety reasons is required to discontinue study drug.