The effect of suppression with antimicrobial agents of the intestinal microflora of paediatric bone marrow graft recipients on severe bacterial and fungal infections and on moderate to severe acute graft-versus-host disease was studied retrospectively. Data on 65 cases of bone marrow transplantation for either severe bone marrow failure or leukaemia, performed in a strict protective environment with either complete or selective gastrointestinal decontamination, were evaluated. All bone marrow grafts were from HLA-identical siblings and were not depleted of T-lymphocytes. Twenty percent of the recipients had one or more episodes of septicaemia during the granulocytopenic period after transplantation, mostly due to gram-positive bacteria. Only five children died due to infection, in each case caused by a microorganism originating from the endogenous flora. Complete gastrointestinal decontamination was superior to selective gastrointestinal decontamination in preventing infectious complications (p less than 0.001). The same was the case for the prevention of acute graft-versus-host disease of grade II or higher, which was observed in 7 of 40 (17.5%) completely decontaminated children versus 9 of 18 (50%) selectively decontaminated children evaluable for graft-versus-host disease (p less than 0.01). It is concluded that complete gastrointestinal decontamination in a strict protective environment is a feasible and very effective method for preventing severe infections and acute graft-versus-host disease after allogeneic bone marrow transplantation in children and adolescents; it resulted in a low transplantation-related mortality of 26% and a good quality of survival in 69% of the graft recipients.