This study is designed to assess the effectiveness of positron emission tomography (PET) with fluorodeoxyglucose in patients with metastatic breast cancer undergoing high dose chemotherapy with stem cell rescue. It includes patients enrolled on two high dose chemotherapy trials (PBT-1 and UPCC #3195). So far, 24 patients have been enrolled in the protocol, and the study is proceeding as planned. We have performed a preliminary analysis of the PET studies of 17 patients performed before high dose chemotherapy. In this subgroup of patients, 11 had active disease demonstrated on the PET study, and 6 had no evidence of metabolically active disease. Three subjects had disease involvement demonstrated only on PET imaging, and in one case, PET confirmed the clinical impression of responding liver metastases shown on CT. Several repeat studies have been performed and will be analyzed when enough prognostic information becomes available. Patient accrual is ongoing and the study is proceeding as scheduled. PET FDG imaging is a promising tool that can potentially predict early therapeutic failure.