A permanent study was established in 1970 and 1971 to measure the impact, spread, and intensification of dwarf mistletoe on precommercially thinned Douglas-fir and lodgepole pine in Montana. Mean diameter growth on all plots was reduced 21 percent in Douglas-fir, but the reduction is only significant at the 0.15 level. No diameter growth reduction occurred in the lodgepole pine. Height growth was not affected in either species. Infected trees, as a percent of the total number of trees in the study, increased from 3.4 percent in 1970 to 9.2 percent in 1983 in Douglas-fir, and from 1.8 percent in 1971 to 7.4 percent in 1984 in lodgepole pine. Dwarf mistletoe rating, a measure of infection intensity, increased in some trees, decreased in some, and did not change in others. There was no effect of thinning at this young age and low dwarf mistletoe intensity. Dwarf mistletoe intensity and impact may increase as the stands mature.