Allogeneic stem cell transplantation (SCT) provides the best mechanism of preventing relapse in acute myeloid leukaemia (AML). However non-relapse mortality (NRM) negates this benefit in older patients. Reduced intensity conditioning (RIC) permits SCT with reduced NRM, but its contribution to cure is uncertain. In the MRC AML15 Trial, patients in remission without favourable risk disease could receive SCT from a matched sibling or unrelated donor (MUD). If aged >45 years, a RIC was recommended and in patients aged 35-44 years, either RIC or myeloablative conditioning was permitted. The aim was to determine which approach improved survival and within which prespecified cytogenetic groups. RIC transplants significantly reduced relapse (adjusted hazard ratio (HR) 0.66 (0.50-0.85), P=0.002) compared to chemotherapy The 5-year overall survival from a sibling RIC (61%) was superior to a MUD RIC (37%; adjusted HR 1.50 (1.01-2.21), P=0.04) due to lower NRM (34 vs 14%, P=0.002) In adjusted analyses, there was a survival benefit for sibling RIC over chemotherapy (59 vs 49%, HR 0.75 (0.57-0.97), P=0.03), with consistent results in intermediate and adverse-risk patients. In patients aged 35-44 years, best outcomes were seen with a sibling RIC transplant, although a comparison with chemotherapy and myeloablative transplant was not significant in adjusted analyses (P=0.3).