Summary:Infectious complications were retrospectively analyzed in 129 transplants, performed in 90 patients, to identify characteristics that qualify breast cancer patients for outpatient-based PBSCT. Thirty-one cases (24%) did not develop fever. Of the remaining 98 cases, 84.7% developed fever during severe neutropenia. On univariate analysis, disease stages II-III, first PBSCT, mucositis grades II-IV and the use of two alkylators were associated with a higher risk of fever development. The latter two factors also affected fever occurrence on multivariate analysis. A longer median time to fever onset was observed in patients conditioned with single as compared to double alkylating agent-containing regimens (respectively 8th vs 6th day, P Ͻ 0.00001). As compared with metastatic breast cancer (MBC), high risk breast cancer showed a 2.3-fold increased risk of developing early fever during neutropenia (CI 2.3-3.8), remaining the only variable still significant on multivariate analysis (P = 0.0039). Combination antibiotic therapy was equivalent to single agent therapy. Patients suffering from microbiologically documented fever were at higher risk of undergoing second-line antibiotic therapy. In conclusion, MBC patients treated with a conditioning regimen containing only one alkylating agent and adequate prophylaxis for mucositis may qualify for outpatientbased PBSCT on the basis of a lower risk of infection. Bone Marrow Transplantation (2001) 28, 883-888. Keywords: breast cancer; stem cell transplant; infection; outpatient High-dose chemotherapy (HDC) followed by peripheral blood stem cell transplantation (PBSCT) is increasingly being used