A fifty-year old patient was treated for acute lymphoblastic leukemia. One month after a complete remission, a syndrome suggesting chronic myeloid leukemia led the authors to study the marrow karyotype which revealed the existence of a Philadelphia chromosome. A second lymphoblastic attack occurred rapidly and a second complete remission was easily obtained. A few weeks later, occurred lymphoblastic meningitis. A new cytogenetic study then showed duplication of the Philadelphia chromosome. One may imagine that the initial attack represented acute lymphoid transformation of chronic myloid leukemia. The theoretical and practical significance of this case is discussed.