We collected data at seven sites in the western US, on the costs of fuel reduction operations (prescribed fire, mechanical treatment, mechanical plus fire), and measured the effects of these treatments on surface fuel and stand parameters. We also modeled the potential behavior of wildfire in the treated and control stands.Gross costs of mechanical treatments were more expensive than those of prescribed fire, but net costs of mechanical treatments after deducting the values of harvested products were, on most sites, less than those of fire.The fire-only treatment reduced surface fuels, while most mechanical treatments (with the probable exception of whole-tree removal) increased these loads. Most mechanical-plus-fire treatments had little net effect on surface fuels. All treatments reduced the number of live trees, on average by about 300, 500 and 700 stems per hectare respectively for fire-only, mechanical, and mechanical-plus-fire. As intended by prescription, the mechanical treatments reduced basal area per hectare significantly. In most cases the fires -- either alone or following mechanical treatment -- killed mostly small trees, having essentially no impact on basal area.The mechanical-plus-fire treatment was the most effective, followed by fire-only, at reducing the modeled severity of wildfire effects under extreme weather conditions. The effectiveness of mechanical-only treatments depended on how much surface fuel remained on site. A whole-tree harvesting system removed the tops and limbs along with the felled trees, thereby reducing potential fire severity more than methods which left slash and/or masticated material within the stands.The various treatments created different conditions, and therefore the treatment intervals needed to maintain desired fire resilience would probably differ as well, being shorter for fire-only than for mechanical-only or mechanical-plus-fire treatments.Decisions about which treatments to prescribe, where, and when, will generally consider not only the financial costs and entry intervals, but other societal benefits and costs of the treatments and of wildfires as well. © 2008 Elsevier B.V. All rights reserved.