Cohorts of 3-6 patients receive escalating doses of lapatinib ditosylate until the maximum tolerated dose (MTD) is determined. The MTD is defined as the dose preceding that at which 2 of 3 or 2 of 6 patients experience dose-limiting toxicity during course 1. At least 6 patients are treated at the MTD. Once the MTD of lapatinib has been determined, patients may be accrued to group B or to a separate pharmacokinetics cohort in group A.