The monitoring of chimerism is a standard procedure to assess engraftment and achievement of full donor lymphoid cells after reduced intensity conditioning (RIC) stem cell transplantation (Allo-SCT). However, there is no consensus on when and how often to monitor post-transplant chimerism. We retrospectively analyzed our experience regarding the impact of acute graft versus host disease (GVHD) for the prediction of allograft chimerism. One-hundred-and-fifteen patients transplanted between 2001 and 2010 were identified. This group included 57 females and 58 males with a median age of 50 years (range: 26–68). Patients evaluated in this study were adult patients with hematologic malignancies, who received transplants from an HLA-matched sibling donor or matched unrelated donor (MUD) at allele level so-called 10/10, and received the RIC regimen including fludarabine/busulfan and anti-thymoglobulin (ATG). Mixed T-cell chimerism was defined as between 5 and 94% recipient cells, and full chimerism was defined as the presence of more than 95% donor T-cell chimerism (TCC). Full donor TCC was achieved in 93 patients (81%) at a median of 77 days (range: 30–120) post-transplant. The cumulative incidence of Grade 2–4 GVHD in our population was 25% (95% CI 17–34). The analysis of the population of patients with acute GVHD grade ≥2 showed that at day 120 after Allo-SCT they all had a total full donor TCC. On the other hand, 78 (68%) patients without acute GVHD grade ≥2 presented with mixed chimerism (p = 0.002) on day 120 post-transplant. Interestingly, patients who received ATG 5 mg/kg obtained a higher probability of complete chimerism compared with those receiving 2.5 mg/kg (p = 0.03). In conclusion, our study demonstrates that acute GVHD was predictive of full donor TCC after RIC Allo-SCT. Therefore, our data may challenge the concept of the frequent or close monitoring of donor chimerism in some patients with ongoing acute GVHD. However, chimerism testing could represent an attractive modality for minimal residual disease detection or for impeding relapse warranting further prospective studies. Am. J. Hematol. 2012. © 2012 Wiley Periodicals, Inc.