In order to evaluate the effect of microaggregate blood filtration on pulmonary status, hemostatic status, and incidence of infection, a prospective study was performed on patients undergoing elective total hip replacement for osteoarthritis. Forty patients were randomized to either a 260 micron standard filter group or a 20 micron microaggregate filter group. Patients were monitored pre- and postoperatively for changes in arterial blood gases and in vitro test of hemostasis. Postoperative measurements were also made of the total volume of blood collected from the operative wound drain and of the number of infections incurred by patients in the two filter groups. Average transfusion was 4.0 units for the standard filter group and 4.6 units for the microaggregate filter group. Results showed that postoperatively, either immediately or after 48 hours, there were no statistically significant differences (p > 0.05) between either filter group for any of the tests of pulmonary or hemostatic function evaluated. For infection no trends were found to suggest that microaggregate filters conveyed any protective effect. These data suggest that routine microaggregate blood filtration of up to 5 units of blood is not required.