Plasma cell leukemia (LP) is a rare ailment that is defined by the presence in the peripheral blood smear of 20% plasma cells or circulating plasma cells that is greater than 2 G / L. There are two variants: the primary form occurring de novo, diagnosed directly in the leukemic phase and the secondary form complicating an already known multiple myeloma. We report in this work a case diagnosed in the laboratory of hematology of the CHU Hassan II of Fez of a multiple myeloma transformed into plasma cell leukemia. It is a rare entity but should not be neglected. Secondary plasma cell leukemia maintains most of the characteristics of multiple myeloma in contrast to primitive plasma cell leukemia. Our work aims to show the common characteristics of the primitive and secondary forms of this leukemia and to discuss the peculiarities of each one.