This is a multicenter open-label phase II trial of trastuzumab in patients with HER2 positive B-ALL in relapse, or with refractory disease. Herceptin is administered as a 4 mg/kg intravenous loading dose followed by 2 mg/kg weekly for at least 2 months and/or till progression. Response and HER2 expression are assessed each month by bone marrow aspirate.