Cytogenetics has an important impact on the prognosis of patients undergoing allogeneic hematopoietic stem cell transplantation (HSCT) for acute myelogenous leukemia (AML) or myelodysplastic syndromes (MDS). However, it is unclear whether currently accepted cytogenetic risk groups, which were established for patients treated mostly with standard therapy, are optimally discriminating for patients undergoing HSCT. Also, the impact of cytogenetics in the growing population of patients with therapy-related disease has not been completely elucidated. In this study, we retrospectively analyzed data on 556 patients with AML or MDS transplanted at our institution. We examined, in multivariate analyses, the contribution of cytogenetics to survival, relapse, and nonrelapse mortality for the 476 patients with de novo disease. We used these results to establish an optimal cytogenetic grouping scheme. We then applied this grouping scheme to the 80 patients with therapy-related disease. Our proposed 3-group cytogenetic classification outperformed the established grouping schemes for both de novo and therapy-related disease. When classified by this new scheme, cytogenetics was the strongest prognostic factor for overall survival in our cohort, through its impact on the risk of relapse (and not on nonrelapse mortality). After accounting for cytogenetics, patients with therapy-related AML or MDS had an equivalent outcome to those with de novo disease. This study demonstrates the impact of cytogenetics on the risk of relapse and death for patients with both de novo and therapy-related disease undergoing transplantation; it also emphasizes the necessity of using cytogenetics to stratify patients entering clinical trials, and provides a system for doing so, which can be validated in a multi-institutional database.