Bone marrow transplantation is frequently used as a consolidation therapy in patients with haematological malignancies to improve the outcome of these patients. Obese individuals have larger absolute lean body and fat masses than non-obese individuals of the same age, gender and height, which might lead to altered pharmacokinetics of chemotherapeutic agents. Data on the impact of body mass on transplant outcome is conflicting. This study included 331 patients (M, 230; F, 101) with 336 allogeneic transplant episodes from two large teaching hospitals in the West Midlands region in United Kingdom. A total of 105 patients had acute myeloid leukaemia, 83 had non-Hodgkin's lymphoma, three had myeloma, 21 had Hodgkin's lymphoma, 34 had acute lymphoblastic leukaemia, 19 had chronic myeloid leukaemia, 22 had chronic lymphocytic leukaemia, 24 had myelodysplasia, seven had T cell non-Hodgkin's lymphoma, six had aplastic leukaemia and seven had myelofibrosis. At transplantation, 40% (N = 133) of the patients had normal and 60% (N = 198) had high body mass index (BMI) with 14% of the patients being obese (BMI >30). After a median follow-up of 24 months (range, 2-79), the mean overall survival (OS) in patients undergoing allograft with normal BMI was 31 months as compared to 39 with high BMI (p:0.06). The mean progression free survival (PFS) in patients undergoing allograft with normal BMI was 33 months as compared to 38 with high BMI (p = 0.13). Of the patients in the high and obese BMI group, 16% developed acute GvHD with 8% grade III-IV and 28% in the normal BMI group with 14% grade III-IV acute GvHD (p = 0.11). Of the patients in the high BMI group, 17% developed chronic GvHD and 30% of the patients in the normal BMI group (p = 0.09). However, higher infection rates and more days of inpatient stay in the first year post-transplant were observed in the high BMI and obese patients, but there was no difference in ITU admissions. This study shows that high BMI and obesity does not adversely impact on either OS or PFS in patients undergoing allogeneic transplantation for haematological malignancies, but it does have a significant impact on infection rates and hospitalisation of high BMI and obese patients. We recommend that patients with high BMI should not be excluded from allogeneic transplantation; however, good supportive care and careful patient selection on the basis of comorbidity index should be undertaken in order to avoid the risks from the increased rates of infection.