One hundred one patients with severe aplastic anemia underwent allogeneic marrow transplantation and received one of three forms of infection prophylaxis: oral nonabsorbable antibiotics and isolation and decontamination in a laminar airflow room (36 patients); prophylactic granulocyte transfusions from a single family member donor (33 patients); or conventional treatment in single rooms with hand-washing and mask precautions (31 patients). During the period of granulocytopenia, patients in the laminar airflow rooms acquired fewer infections than either of the other groups, but this difference was statistically significant only when compared with the group receiving conventional treatment. Patients in the laminar airflow rooms had significantly fewer infections after engraftment as compared with the other two groups. Incidence of interstitial pneumonia and graft rejection was not different among the three groups. Acute graft-versus-host disease occurred later (Day 47) in the group in the laminar airflow rooms as compared with the group receiving prophylactic granulocyte transfusions (Day 23) or the group receiving conventional treatment (Day 20). The incidence of grades II to IV acute graft-versus-host disease was less in the patients in the laminar airflow rooms but only reached borderline significance (p = 0.08) when compared with the conventionally treated patients. The survival at Day 100 was 92 percent for the group in the laminar airflow rooms, 79 percent for the group receiving prophylactic granulocyte transfusions, and 64 percent for the group receiving conventional treatment. © 1984.