* Information on the genetic variation of plant response to elevated CO(2) (e[CO(2)]) is needed to understand plant adaptation and to pinpoint likely evolutionary response to future high atmospheric CO(2) concentrations. * Here, quantitative trait loci (QTL) for above- and below-ground tree growth were determined in a pedigree - an F(2) hybrid of poplar (Populus trichocarpa and Populus deltoides), following season-long exposure to either current day ambient CO(2) (a[CO(2)]) or e[CO(2)] at 600 microl l(-1), and genotype by environment interactions investigated. * In the F(2) generation, both above- and below-ground growth showed a significant increase in e[CO(2)]. Three areas of the genome on linkage groups I, IX and XII were identified as important in determining above-ground growth response to e[CO(2)], while an additional three areas of the genome on linkage groups IV, XVI and XIX appeared important in determining root growth response to e[CO(2)]. * These results quantify and identify genetic variation in response to e[CO(2)] and provide an insight into genomic response to the changing environment.