Reverse isolation and prophylactic oral nonabsorbable antibiotics were evaluated among 64 consecutive noninfected adults with acute nonlymphocytic leukemia admitted for remission induction. Patients were randomly allocated to laminar air flow room reverse isolation with oral nonabsorbable antibiotics (LAF + A), routine hospital ward care with antibiotics (W + A), or ward care alone (W). The LAF + A patients had a significantly decreased incidence of total infection, bacteremias, pneumonias, rectal abscesses, urinary tract infection, and pharyngitis. Infectious deaths were reduced in the LAF + A group and the time to the first infection or to fatal infection was delayed. The W + A patients who regularly ingested the antibiotics had a reduction in infections similar to that of the LAF + A patients but those who could not tolerate the antibiotics had an incidence of infection comparable to the ward patients. The LAF + A and the W + A patients also had higher complete remission rates and longer median survival than the unprotected ward patients.