IL-17 is involved in inducing and mediating pro-inflammatory responses. The association of IL-17 with tumor growth or graft-versus-host disease (GVHD) has become a subject of controversy. We hypothesized that serum IL-17 (sIL-17) levels during the peri-transplant period may affect alloreactive responses after allogeneic stem cell transplantation (SCT). sIL-17 levels of 95 patients with leukemia who had undergone myeloablative allogeneic SCT were measured using ELISA before conditioning and on day 0, +7, and +14 after transplantation. With a median follow-up of 17 months, the overall survival, disease-free survival, non-relapse mortality, and relapse incidence were 70.9%, 66.3%, 10.3%, and 23.4%, respectively. Ten patients relapsed within 180 days (early relapse, 10.5%) post-transplant. The cumulative incidence of acute GVHD over grade II and chronic GVHD was 55.8% and 69.0%, respectively. Analyses using repeated measures of ANOVA and mean values of sIL-17 revealed that patients relapsed within 180 days had higher sIL-17 levels, whereas no association existed between sIL-17 levels and other clinical outcomes, including acute GVHD. Receiver operating characteristic curve analyses also revealed that sIL-17 levels were available for the prediction of early relapse and that patients with higher sIL-17 levels at each time point had a significantly higher early relapse. Multivariate analyses and subgroup analyses with only standard disease status suggest the association of sIL-17 levels with subsequent early relapse independent of disease status at transplantation. This study is the first one demonstrating the early change in sIL-17 during the peri-transplant period and the association with early relapse in humans.