T-cell reconstitution after allo-SCT initially depends on homeostatic peripheral expansion of donor T cells, the level of which may promote the differentiation of alloreactive and tumor-reactive effectors. IL-7 and IL-15 exert their effect as key homeostatic cytokines. We prospectively investigated plasma levels of IL-7 and IL-15 in a homogeneous group of 40 patients in CR of their hematologic malignancy undergoing myeloablative, fully (10/10) HLA-matched BMT. IL-7 and IL-15 proceeded along similar kinetic courses, peaking at wide ranges (3.8–30.2 and 14.3–66 pg/ml, respectively) on day+ 14 when all patients were profoundly lymphopenic. Occurrence and grade of subsequent acute GVHD were significantly associated with heightened day+ 14 IL-7 and IL-15 levels. Association of peak IL-7 level to grade 2–4 acute GVHD was confirmed by Cox multivariate analysis (hazard ratio (HR)= 5.38; P= 0.022). Malignancy relapse was significantly associated with reduced day+ 14 levels of IL-15 (Cox multivariate analysis: HR= 0.93; P= 0.035). Plasma IL-7 and IL-15 levels in the early post transplantation period are therefore biomarkers that can help predict subsequent development of acute GVHD and malignancy relapse.