60 adult patients with acute leukemia (AL) previously untreated or in relapse received induction chemotherapy with different types of supportive care. Group A was decontaminated by nonabsorbable antibiotics in strict reverse isolation, group B was isolated only and group C was treated under routine hospital conditions. There were less infections in group A and B compared with group C. 76% of patients in group A achieved remission, in contrast to 57% in group B and 59% in group C. Late evaluation three years after termination of the study showed that all patients were dead except four patients being in first remission after discontinuation of maintenance treatment. Three patients were treated in group A, one patient in group C. However, the differences of survival and remission rates were not significant. Failure to demonstrate significant advantage of isolation and decontamination in treatment of AL was caused by insufficient suppression of microbes by the applied antimicrobial measures. Thus, it is suggested to investigate better antimicrobial treatment before gnotobiotic care is accepted as routine supportive treatment in AL.