OBJECTIVES: To explore whether combination therapy (COMBO) with dutasteride and tamsulosin, provides meaningful additional benefits compared to each individual therapy or watchful waiting (WW). METHODS:   COMBAT,  a randomized, double-blind, parallel group study investigating the efficacy and safety of dutasteride (0.5mg) and tamsulosin (0.4mg), administered once daily for four years, alone and in COMBO, in symptomatic BPH patients.  A Markov model was developed comparing the monotherapies, COMBO, and a watchful waiting (WW) reference group.  The model evaluated the treatment costs and quality adjusted life years (QALY) at four yrs and lifetime based on the efficacy results of COMBAT.  Incremental cost effectiveness ratios (ICER) were calculated using WW as the basis of comparison.  One way sensitivity analyses were conducted in addition to the basecase analysis. RESULTS:  Four years: ICER results for the COMBO and dutasteride arms were similar and higher than the tamsulosin arm, as the reduction in BPH progression events could not offset the higher drug costs.  Overall, the QALY benefits of the COMBO and dutasteride arms were greater than the tamsulosin arm. Lifetime:  ICERs decrease from those estimated at 4 yrs.  COMBO benefits become apparent in this extended time horizon.  Follow-up and BPH progression costs of the monotherapies outweigh the higher initial drug cost of COMBO, in addition monitoring costs are lower for COMBO than for either of the MONOs or WW.  The difference in event costs is indicative of lower BPH progression events in COMBO arm. CONCLUSIONS: Results at both time horizons indicate that the combination of dutasteride with tamsulosin for BPH provides gains in QALYs over WW and monotherapy arms, and acceptable cost effectiveness ratios compared to WW and the monotherapy arms.   The availability of a fixed dose COMBO, priced the same as dutasteride monotherapy could provide substantial healthcare benefits to patients and cost savings to the healthcare system.