An adult patient with acute lymphoblastic leukemia associated with a 14q+ marker chromosome is presented. The abnormality resulted from a translocation of material from the long arm of chromosome 11. The leukemic cells were found to be B cells on the basis of surface immunoglobulins, lack of receptors for sheep erythrocytes, and a characteristically low level of adenosine deaminase activity. In other patients with ALL studied by us or reported by others in whom chromosome banding was done, a 14q+ chromosome was present in only one instance, also a case of B cell ALL. These two cases are the only examples of B cell ALL studied with chromosome banding reported to date. The frequent occurrence of a 14q+ chromosome in other malignant lymphoproliferative diseases of B cell origin suggests that a general association may exist between the 14q+ abnormality and B cell neoplasms. Cytogenetic analysis may therefore be useful in defining subtypes of ALL and in relating specific chromosomal abnormalities to lymphoproliferative disorders.