The rate of upward advance and intensification of dwarf mistletoe on immature coastal western hemlock were determined by reconstructing the infection history of individual trees from data on age, height above ground, and sex of mistletoe infections present at the time of observation. The vertical rate of spread was 30 ± 4 cm/year in a dense stand (750 trees per hectare, 19 m in height) and 65 ± 4 cm/year in an open stand (250 trees per hectare, 26 m in height). This compares with height growth of the trees of 33 and 58 cm/year in the two stands, respectively. The number of new infections per year increased logarithmically with time, doubling roughly every 4 years. An asymtotic stabilization of the rate of intensification over the last 6 years probably reflects crown closure. The results suggest that dwarf mistletoe may not be a serious threat to thrifty, immature, dense, coastal hemlock stands in southern British Columbia, unless such stands are overtopped by infected residual trees.