Relapse is a major cause of failure after allogeneic hematopoietic cell transplantation (HCT) in patients with myelodysplastic syndromes (MDS). We analyzed the relapse pattern in 1007 patients who underwent transplantation for MDS to identify factors that may determine the timing of relapse. Overall, 254 patients relapsed: 213 before 18 months and 41 later than 18 months after HCT, a time point frequently used in clinical trials. The hazard of relapse declined progressively with time since transplantation. A higher proportion of patients with early relapse had high-risk cytogenetics compared with patients with late relapse (P = .009). Patients with late relapse had suggestively longer postrelapse survival than patients who relapsed early, although the difference was not statistically significant (P = .07). Among 41 late relapsing patients, sequential cytogenetic data were available in 36. In 41% of these, new clonal abnormalities in addition to pre-HCT findings were identified at relapse; in 30% pre-HCT abnormalities were replaced by new clones, in 17.3% the same clone was present before HCT and at relapse, and in 9.7%, no abnormalities were present either before HCT or at relapse. Comparative chromosomal genomic array testing in 3 patients with late relapse showed molecular differences not detectable by cytogenetics between the pre-HCT clones and the clones at relapse. These data show that late relapses are not infrequent in patients who undergo transplantation for MDS. The pattern of new cytogenetic alterations at late relapse is similar to that observed in patients with early relapse and supports the concept that MDS relapse early and late after HCT is frequently due to the emergence of clones not detectable before HCT.