Complete microbial decontamination (laminar air flow room, sterile nursing and oral administration of cefamandole, gentamicin and nystatin) was carried out in 65 consecutive patients prior to allogeneic BMT for leukaemia (n = 58) or aplastic anaemia (n = 7). Very few microorganisms persisted during the post-transplant treatment period, and the gut became sterile in all except for Candida in 11 patients. Six uncomplicated septicaemias, all with persistent organisms simultaneously present in the mouth (Pseudomonas 3, Serratia 1, Candida 2) occurred during a total of 1,360 days with granulocyte counts less than 0.5 X 10(9)/l. Post-transplant fever occurred in 52 patients, exceeding 40 degrees C in 25. Guided by the surveillance cultures only 46% of 43 unexplained febrile reactions were treated with systemic antimicrobials. Significant acute graft versus host disease (AGVHD) occurred in 14 (27%) of 52 patients receiving standard prophylaxis and HLA-matched grafts; immunosuppressive treatment was needed in 8 cases (16%). Thus, the additional costs of total microbial decontamination appear partially regained by a decreased morbidity and a reduced need for antimicrobial and immunosuppressive treatment, although neither fever nor AGVHD could be prevented.