We present a case of 3-week-old infant, in whom we established the diagnosis of congenital acute lymphoblastic leukemia (ALL), characterized by the immunophenotype of the most immature B-cell precursors (pro-B-ALL) and chromosomal translocation t(4;11) associated with the rearrangement of MLL and AF4 genes. The clinical course of this leukemia deserves particular attention, because it was different from classical presentation: diagnosis--not immediately after birth--but at the age of 3 weeks, no signs of hemorrhagic diathesis, lack of nodular skin infiltrates, and no central nervous system involvement. Exchange blood transfusions performed during the remission induction treatment saved the child from life-threatening symptoms of leukostasis at the initial stage of leukemia. Despite successful remission induction in bone marrow, the infant relapsed during the fifth month of treatment. After subsequent chemotherapy course the child died owing to fatal infectious complications. Despite enormous progress in treatment of childhood ALL, this type of leukemia in infants below 6 months of age, and particularly in cases of congenital leukemia is still associated with a very poor prognosis.