Western hemlock looper (Lambdina fiscellaria lugubrosa) populations along coastal British Columbia experienced outbreak levels from 2000 to 2003, leading to visible defoliation of western hemlock (Tsuga heterophylla) dominated forests. This study represents the first detailed ecological assessment of forest compositional and structural response to western hemlock looper defoliation in coastal BC forests. Stands severely defoliated for multiple years underwent massive mortality approaching 100% of canopy and subcanopy trees. Smaller trees experienced greater mortality than larger trees. A shift in living canopy composition occurred as western hemlock and Douglas-fir trees died at a greater rate than western redcedar or amabalis fir. Canopy openness increased from 5.2% in lightly defoliated sites to 23.7% for severely defoliated sites. Understories shifted in structure and composition, as the amount of bare ground and tree cover decreased and the amount of cover of shrubs, ferns and herbaceous species increased with increasing severity of canopy defoliation. Shrub cover (24.9%), fern cover (27.6%) and herbaceous species cover (8.7%) was significantly higher in severely defoliated sites than in lightly defoliated sites (16.7%, 13.3% and 1.0% respectively). Understory species richness was also greater in severely defoliated sites compared to lightly defoliated sites, with a greater number of shrub species (2.22 species per plot compared to 3.53), fern species (1.60 to 2.42) and herbaceous species (0.29 to 1.00). Therefore, severe western hemlock looper outbreaks shift both forest composition and structure by causing extensive mortality of canopy species and the establishment of a non-tree dominated understory. Frequent western hemlock looper outbreaks represent an important natural disturbance in coastal BC forests that increase landscape heterogeneity.