Abstract The aspen hybrid breeding program has been active in the Lake States since 1955. This study summarizes long-term performance of aspen hybrids in six locations in Wisconsin and Upper Michigan. Interspecific hybrids of native bigtooth aspen and quaking aspen with other poplar species show improved growth when compared with native aspen species across a range of site quality. Quaking aspen hybrids with Populus tremula (XT-Ta) were the most promising based on growth, wood quality, and disease susceptibility. Individual tree volume growth for the XT-Ta hybrids was more than double that of progeny of pure quaking aspen parents (XT). Other aspen hybrids with P. davidiana, P. alba, and P. canescens parentage exhibited special growth potential on relatively poor and dry-sites, but were susceptible to a bronze leaf disease. Hybridization was more effective in improving aspen growth than breeding within pure species. Hybrid superiority was consistent across a range of site indexes, and showed the greatest potential on high-quality sites. Genetic gains for the XT-Ta hybrids, based on 15-yr height measurements, ranged from 29 to 34% across sites. Gain in volume growth is expected to be over 100%. Short-rotation (20-yr) commercial plantings with aspen hybrids are recommended on low-to medium quality northern hardwood sites. North. J. Appl. For. 10(3): 117-122.