Multiple myeloma (MM) patients have an 11-fold increased risk of developing myeloid neoplasms compared to the general population; however, acute lymphoblastic leukemia (ALL) is rarely observed. Given that both MM and the majority of ALL are of B cell origin, this raises the question of whether ALL in patients with MM arises from the same clone. We report 13 cases of B-cell ALL following therapy for MM. The interval from MM diagnosis to ALL onset was 5.4 years (range 3.3-10). The median age at the time of ALL diagnosis was 60 years (range 43-67). MM therapy included immunomodulatory agents in all patients and autologous hematopoietic cell transplantation in 10 (77%) patients preceding ALL diagnosis. ALL genetics showed a normal karyotype, TP53 mutation/deletion, and monosomy 7 or 7q deletion in 5, 3, and 2 cases, respectively. Analysis of paired samples of MM and ALL using whole exome sequencing demonstrated that the malignancies arose from different clones. Thus, ALL as a second primary malignancy following MM is not clonally related but could potentially represent a therapy-related leukemia.