Infiltration of the breast by acute leukemia is extremely rare. It is known that the relapse of acute lymphoblastic leukemia outside blood and hematopoietic tissue usually occurs in the central nervous system and testicles. Relapses have been reported in other sites following transplantation and, when this occurs, the prognosis is poor. However, a relationship has been observed between young pregnant women and acute lymphoblastic leukemia relapse in the breast as a characteristic site. Extramedullary relapses are more common in acute myelogenous leukemia, and this is why we are presenting this case report. Case report: A female patient of 22 years of age with B phenotype acute lymphoblastic leukemia diagnosed in September 2009, received an induction chemotherapy regimen based on prednisone, vincristine and daunorubicin from 2009 to 2012. A complete hematologic response was adquired. In March 2013 a bone marrow relapse was documented. At that time she was 16 weeks pregnant. She received an alternative regimen of Larson (Cancer and Leukemia Group B); (Induction with: cyclophosphamide, daunorubicin, vincristine, prednisone, and asparaginase. Early intensification: methotrexate, cyclophosphamide, mercaptopurine, cytarabine, vincristine, asparaginase. Central nervous system prophylaxis: with methotrexate. Late intensification: doxorubicin, vincristine, dexamethasone, cyclophosphamide, biguanide, cytarabine. Maintenance: vincristine, prednisone, methotrexate, mercaptopurine). At 26 weeks of pregnancy she received treatment with two cycles of cytarabine 75 mg/m2/day for four days and central nervous system prophylaxis with cytarabine 40 mg and dexamethasone 8 mg as re-induction regimen. The pregnancy was interrupted at 36 weeks, resulting in a male child weigthing 2,985 g, APGAR score 8/9, without apparent complications. The patient later attended the consultancy with neurological disorders and a nodule in the upper external quadrant of the left breast that was painful on palpation and so a diagnosis protocol was initiated. Conclusions: An associattion is observed between pregnancy, breastfeeding, and the acute lymphoblastic leukemia infiltration into the mammary gland. The association and the genetic molecular profile should be describe.