Mountain pine beetle (Dendroctonus ponderosae Hopkins) (MPB), can cause extensive ponderosa pine (Pinus ponderosa Dougl. ex Laws.) mortality in the Black Hills of South Dakota and Wyoming, USA. Lower tree densities have been associated with reduced MPB-caused tree mortality, but few studies have reported on large-scale thinning and most data come from small plots that may not be representative of a large area. We quantified MPB-caused tree mortality in 21 pairs of commercially thinned and unthinned stands across the Black Hills. Both pre- and postoutbreak, unthinned stands had higher ponderosa pine basal area, tree density, and stand density index. Quadratic mean diameter was larger in thinned stands, both pre- and postoutbreak. Percent ponderosa pine basal area and tree density killed by MPB in unthinned stands were 38.2 +/- 7.0 and 34.4 +/- 6.9% compared with 3.9 +/- 3.2 and 3.6 +/- 2.9% in thinned stands, respectively. All stands were thinned within 2 years of exposure to MPB, suggesting a rapid effect from thinning treatments in mitigating tree mortality attributed to MPB. Stand density reductions through silviculture across a large geographical area can abate MPB-caused tree mortality.