Inclusion criteria:

• May have received prior lapatinib in another trial. Previous capecitabine (as previous agent or non-lapatinib containing regimen) is also permitted.
• Prior treatment with hormonal therapy is allowed.
• Must have advanced or metastatic breast cancer with progression (as assessed by modified RECIST) after prior therapy, which must include all of the following: prior treatment with an anthracycline, a taxane, and trastuzumab alone or in combination with other therapy. Trastuzumab must have been administered in the adjuvant, or locally advanced or metastatic setting.
• Must have tumors that overexpress ErbB2 defined as +3 by IHC or FISH positive for ErbB2 gene amplification.

The status of ErbB2 expression must be documented prior to study entry.

• Must be >/= 18 years of age
• Life expectancy of > 8 weeks
• Must have recovered from side effects of previous treatment
• Patients with CNS mets are eligible provided treatment with prohibited medications as listed in the protocol are not required
• Cardiac ejection fraction must be within the institutional range of normal as measured by ECHO. MUGA scans are allowed if ECHOs cannot be performed
• Able to swallow and retain oral medications
• Must have adequate hematologic, hepatic and renal function

Exclusion criteria:

• Pregnant or lactating females
• Malabsorption syndrome, disease significantly affecting GI function, or resection of the stomach or bowel, or ulcerative colitis
• Concurrent disease or condition that would make the patient inappropriate for study participation
• Unresolved or unstable toxicity from prior administration of another investigational drug and/or prior cancer treatment
• Uncontrolled infection
• Active cardiac disease defined as history of uncontrolled or symptomatic angina; history of arrhythmias requiring medication, or clinically significant, with the exception of asymptomatic atrial fibrillation requiring anticoagulation; MI < 6 months from study entry; uncontrolled or symptomatic CHF; ejection fraction below the institutional normal limit; any other cardiac condition that would make this protocol unreasonably hazardous for the patient
• Receiving concurrent chemotherapy (other than capecitabine), radiation therapy, immunotherapy, biologic therapy or hormonal therapy for cancer. Concurrent therapy with bisphosphonates is allowed
• History of allergic reaction attributed to compounds of similar composition to lapatinib or any excipients
• History of allergic reactions attributed to compounds of similar chemical composition to capecitabine, fluorouracil or excipients
• Known DPD deficiency