The impact of filgrastim on the use of health care resources during recovery from autologous bone marrow transplantation (ABMT) was studied. The charts for patients with metastatic breast cancer treated with ABMT at a general hospital between November 1989 and July 1993 were reviewed by Blue Cross of Western Pennsylvania. The 58 patients were divided into five groups: group 1-bone marrow purged, no filgrastim therapy; group 2-bone marrow not purged, no filgrastim therapy; group 3-bone marrow purged, filgrastim therapy after ABMT; group 4-bone marrow not purged, filgrastim therapy after ABMT; and group 5-peripheral blood stem cells (PBSCs) given, followed by filgrastim therapy. The groups were compared for total length of stay (LOS), number of days the absolute neutrophil count (ANC) was < 500/cu mm, total number of days of filgrastim therapy, and total number of cumulative unit days of antimicrobial use. Total LOS was shorter for patients who received filgrastim (groups 3-5) than for patients who did not. Filgrastim was associated with fewer days of ANC < 500/cu mm in groups 4 and 5. The total number of cumulative unit days of antimicrobial use was lower in filgrastim recipients. Patients who received PBSCs needed fewer days of filgrastim therapy than the other filgrastim recipients. The health insurance company determined that, as a result of filgrastim therapy and PBSC transplantation, ABMT costs to the company have dropped by more than 50% since 1990. Patients now have available an alternative to conventional therapy for metastatic breast cancer without prejudice or penalty from their payer.