The biological basis for acute leukemia associated with mediastinal germ cell tumors has remained unexplained due to lack of critical data that would illuminate the genetic relationship between the two tumors in a given patient. Here we present results of serial cytogenetic investigations on the pretreatment and posttreatment mediastinal yolk sac tumor and immature teratoma biopsies and two separate leukemic bone marrow aspirates from a patient who developed acute nonlymphocytic leukemia 11 months after the initial diagnosis of the germ cell tumor. Presence of an i(12p) in all tumor clones and trisomy 21 in one clone in the posttreatment mediastinal tumor and all leukemic clones establishes the common origin of all tumor clones and shows that in this case the leukemia was derived from the malignant germ cell clone.