Lymphomas secondarily involving the breast are uncommon, although they do represent the largest group of tumors metastatic to breast. A 20-year-old female with lymphoblastic lymphoma (LBL) presented here with 3 month history of weight loss, night sweats, fatigue and a mass in her left breast. Her physical examination revealed a left breast mass, lympadenopathy, bilateral pleural effusion and hepatomegaly. WBC count was 17,710/mm(3) and LDH was mildly elevated. Breast ultrasound showed a 1.7 cm mass in the inner lower quadrant of left breast. Biopsy of the breast mass showed diffuse infiltration with small, round atypical cells which did not stain with CD20, CD43, CD34, cytokeratine and were positive for CD3. She was diagnosed as leukemic phase of a precursor T-cell LBL and treated with 6 cycles of CHOP ( cyclophosphamide, doxorubicin, vincristine and prednisone), intrathecal methotrexate and cranial radiotherapy, achieving a complete response. She then was started on maintenance therapy. Four months later she returned with CNS involvement and was started on induction treatment. She had a very aggressive course of disease and died only 12 months after diagnosis. Breast involvement is very rarely seen in precursor T-cell LBL/ALL and in this patient occurred secondarily as part of widespread disease.