Early absolute lymphocyte count (ALC) recovery at day 15 post-autologous stem cell transplantation (ASCT) is a powerful prognostic indicator for survival in multiple myeloma and non-Hodgkin's lymphoma. The relationship of ALC with clinical outcomes in metastatic breast cancer is unknown. We evaluated all 29 patients with metastatic breast cancer who underwent ASCT at the Mayo Clinic, Rochester, Minnesota, from 1994 to 1999. The ALC threshold was set at 500 cells/microl on day 15 post-ASCT based on previous experience with hematologic malignancies. All patients were followed for a minimum of 2 years or until death, with a median follow-up for living patients of 2.25 years. Of the 29 patients, 17 have died with disease progression, two are alive and have progressed, and 10 are alive without progression. The median overall and progression-free survival times were significantly better for the 20 patients with ALC > or = 500 cells/microl compared with the nine patients with ALC <500 cells/microl (not reached vs 14 months, P < 0.0001; 24 vs 7 months, P < 0.0015, respectively). In conclusion, ALC > or = 500 cells/microl on day 15 post-ASCT was associated with significantly better survival in patients with metastatic breast cancer, suggesting the importance of early immune recovery post-ASCT in these patients.