The clinical effectiveness of bioclean rooms was studied in relation to prevention of infection during treatment for induction of remission of acute leukemia. According to the steps in clean patient care, patients were divided into three groups, Group A consisting of 106 patients who were accommodated in laminar air flow (LAF) rooms under strictly clean nursing, Group B consisting of 99 patients who were treated in disinfected rooms with LAF without any clean nursing techniques and Group C consisting of 188 patients who stayed in conventional wards. Since all patients were treated with antileukemic drugs and preventive administration of antimicrobial drugs under the same regimen, any difference between groups should be attributable to LAF or LAF and clean nursing. A marked and stepwise reduction in the incidence of infection between Groups A, B and C was observed. A statistically significant reduction in pneumonia, upper respiratory tract infection, periproctitis and skin abscesses was also observed in Groups A and B. As to the causative organisms, a relative increase in incidence of infections caused by Pseudomonas aeruginosa and Escherichia coli and a decrease in those caused by Proteus and fungi were observed in Groups A and B, as compared with Group C. These data suggest that pneumonia etc. may be exogenous infections and that Proteus and fungi are exogenous organisms, while Pseudomonas aeruginosa and Escherichia coli are endogenous bacteria. These data seem very helpful for setting up several steps in bioclean rooms for effective and economic patient care.