Summary. Acute graft-versus-host disease (GvHD) is an important complication of bone marrow transplantation in humans. Risk factors are imprecisely defined and controversial. We analysed data from 2036 recipients of HLA-identical sibling transplants for leukaemia or aplastic anaemia to identify risk factors for GvHD. Analyses indicate that grading of GvHD can be reproducibly divided into absent or mild versus moderate to severe; 2-year actuarial probability was 54% (95% confidence interval 52–56%) for absent or mild and 46% (44–48%) for moderate to severe. Factors predictive of development of moderate to severe GvHD include donor/recipient sex-match (female→male greater than others, relative risk 2.0, P<0.001). This risk was markedly increased if female donors for male recipients were previously pregnant or transfused (relative risk 2.9, P<0.0001). Older patients were at increased risk of GvHD (relative risk 1.6, P<0.001), but the age gradient was modest, even the youngest patients had a substantial risk of GvHD and, if parous or transfused female→male transplants were excluded, age was not a significant risk factor. Cyclosporine or methotrexate were equally effective at preventing GvHD and were superior to no prophylaxis (relative risk 2.3, P<0.01). These data should be useful in estimating the risk of acute GvHD in an individual patient and in designing clinical trials to investigate methods to modify or prevent GvHD.