A double-blind study was conducted to determine the effectiveness of 5-mum final filtration in preventing phlebitis secondary to intravenous fluid administration. An experimental administration set which contained distal portion of the tubing was used for patients randomly assigned to receive filtered intravenous solutions. An identical administration set minus the filter was used in the control group to allow double-blind evaluation of the injection site. Data were collected on 49 patients whose average age was eight years and one month. Most patients had a primary diagnosis of acute lymphocytic or myelocytic leukemia or solid tumor. The patients' injection sites were evaluated daily for clinical findings of phlebitis (erythema, induration, heat, erythematous-streak and discomfort). The incidence of phlebitis was significantly lower (p less than 0.01) in patients receiving filtered intravenous solutions. For the patients in this study, filtration of intravenous fluids with a 5-mum mesh filter appeared to be a feasible and effective means of reducing postinfusion complications.