Inclusion Criteria:

- Patients with histologically or cytologically proven metastatic breast cancer.

- Patients with either estrogen or progesterone receptor positivity on the most
recently examined tumor biopsy.

- Patients must have most recently been using an anti-estrogen (tamoxifen, toremifene,
raloxifene, or fulvestrant) or an aromatase inhibitor.

- Patients must have had either a partial response or better, or stable disease for 24
weeks or longer, followed by disease progression, on the current or most recent
hormonal therapy for management of metastatic breast cancer.

- Patients must be enrolled within six weeks of defining disease progression on
hormonal therapy.

- Patients must have stopped fulvestrant at least four weeks prior and other endocrine
therapy at least two weeks prior to enrollment on study.

- Patients must have either measurable disease or at least one evaluable bone lesion
that has not been irradiated. Measurable disease is not necessary.

- Estimated life expectancy of at least 6 months.

- ECOG performance status 0-2.

- Adequate hematologic, hepatic, and renal function.

- Patients must be post-menopausal, or they must be practicing either abstinence or an
adequate method of contraception, or their sexual partner must be sterile.

- All patients must be able to swallow, retain, and absorb oral medications.

- All patients must be able to give informed consent indicating that they are aware of
the investigational nature of this study.

Exclusion Criteria:

- Patients may not have received an investigational agent within the prior four weeks.

- Patients may not have received trastuzumab within three weeks of study entry.

- Patients may not have had major surgery within the prior two weeks.

- Patients may not have Class III or IV heart failure as defined by the NYHA functional
classification system.

- Patients may not have a left ventricular ejection fraction < 40% based on MUGA or
echocardiogram.

- Patients may not have uncontrolled brain metastases or leptomeningeal disease.

- Patients may not have rapidly progressive visceral metastases.

- Patients may not have a serious illness or conditions including clinically
significant cardiac disease, angina pectoris, serious psychiatric disorder, or an
active infection.

- Patients may not be receiving concurrent medications (listed in the protocol) which
may interact with lapatinib during treatment with lapatinib.