Historically eosinophilia has been associated with chronic GVHD. But it is not clear what role eosinophilia plays in cGVHD and if it can be used as a cGVHD marker. Oftentimes cGVHD onset is insidious, and easily missed. We wanted to see if eosinophil percentage in peripheral blood could be used as a predictive marker of cGVHD. We retrospectively analysed 170 allogeneic stem cell transplants (allo-SCTs) that were performed at UMASS Medical Center from Jan 1st2009 until June 30 2015. Two patients had second allo- SCT from the same donor as first, one of whom developed cGVHD both times and the other developed it the second time. Allo- SCTs included matched and mismatched, related and unrelated adult, and cord blood SCTs. GVHD prophylaxis regimens used were Tacrolimus /cyclosporine/sirolimus in conjunction with mycophenolate and post transplant cyclophosphamide (PTC) in conjunction with sirolimus. Anti thymocyte globulin was a part of gvhd prophylaxis for all patients except for fully matched recipients who received PTC. The % of eosinophils in the periphal blood white cell count differential was analysed. Any elevation over 8% (lab cut off) was considered eosinophilia. Absolute eosinophil count was not used as it is dependent on the absolute white blood cell count, recovery of which can be variable post transplant. Incidence of cGVHD was 27% (46 out of 170). Median age at transplant of patients who developed cGVHD was 54 yrs (24 to 84) while that of the allo-SCT cohort was 59 (18 to 84) yrs. Average time to development of cGVHD was 250 days (89-684). Thirty one (67%) had multi organ involvement. Fifteen (33%) had single organ involvement. Skin was the most common single organ involved (47%). Rate of relapse in the cGVHD cohort was similar between CGVHD and allo SCT cohort, 17% vs 16% respectively. Forty two (91%) had eosinophilia associated with cGVHD. All four patients, who did not have any eosinophilia, developed cGVHD between day 100 and day 180 (110,112,120 and 154 days) after transplant. Three of the four patients without eosinophilia, who died, had a short median survival (183,187,246 days). Eosinophilia preceded the diagnosis of cGVHD in all transplants except for one. On the average eosinophilia preceded the diagnosis of cGVHD by 156 days (2 to 482). Survival of patients who developed esoinophilia prior to cGVHD was much better at 63% compared to 44% for the entire allo-SCT cohort. Eosinophilia in cGVHD turned out to be a good prognostic marker in our study similar to what has been reported in previous studies. In the presence of adequate count recovery and in the absence of another indication for eosinophilia such as, known drug eruption, graft failure etc, eosinophilia can be a very useful predictive marker that typically precedes the development of cGVHD. A high index of suspicion in the presence of eosinophilia may be helpful in minimizing delay in the diagnosis of cGVHD.