A history, physical examination, blood counts, and chemistries will be done at baseline, and at regular intervals through the course of the study. A CT scan and bone scan will be done prior to treatment and at weeks 14 and 26. Assays for plasma DNA will be performed on blood sampled at baseline and at multiple time points throughout the course of treatment. Percutaneous biopsies will be taken in selected patients with accessible disease, 72 hours or less prior to the start of lapatinib, and again 13-15 days, and 27-29 days following the start of lapatinib. The day 13-15 biopsy will be done just prior to the resumption of the patient's endocrine therapy. Assays for phospho-ERK, phospho-Akt, Cyclin D1, Ki-67, and IRS-1 will be performed by conventional immunohistochemistry on the biopsied tissue.