A t(11;14)(p15;q11) was the sole chromosome abnormality observed in the malignant cells of a 10-year-old boy with acute leukemia. Morphologically, these cells were classified as L1 by the criteria of the French-American-British Working Group. Cytochemical analysis revealed that the leukemic cells were negative for Sudan Black B, periodic acid Schiff, and esterases, and positive for acid phosphatase. Immunophenotyping disclosed that the cells expressed a very immature antigenic profile [CD34+, CD7+, cytoplasmic CD3+, membrane CD3-, CD4-, and CD8-]. In spite of very intensive chemotherapy, complete remission was never induced, and the child died of progressive disease. The relationship of this case to other reported cases of acute leukemia arising from immature pluripotent hematopoietic cells is discussed.