Ninety-four unmanaged lodgepole pine stands, from a broad geographical range in the western United States, were examined to evaluate the relationship between stand density and susceptibility to mountain pine beetle (MPB),Dendroctonus ponderosaeHopkins, attack. Population trends were not significantly related to variation in stand density as measured by stand density index (SDI). Percentage of trees killed per hectare by MPB in stands with>80% lodgepole pine did vary significantly with changes in SDI. From these data three SDI zones were identified as follows: 1) stands with SDI's of<125 showed low potential for attack, 2) stands between 125 and 250 SDI showed much greater levels of tree mortality, gradually decreasing toward the 250 SDI, 3) tree mortality decreased in stands as density increased beyond the 250 SDI value.