Trisomy 13 occurring as a single cytogenetic abnormality has been associated with undifferentiated or biphenotypic acute leukemias and with an adverse prognostic outcome. We describe for the first time a case of B-cell common acute lymphoblastic leukemia (ALL) with trisomy 13 at diagnosis in an 18-year-old boy. The leukemic cells did not express myelocytic or T-cell associated antigens and no molecular abnormalities were detected. Following treatment, according to the GIMEMA ALL 0496 protocol, the patient achieved a brief (2 months) complete remission. At relapse, cytogenetic analysis showed karyotypic evolution that included two novel subclones carrying a del(6q), a del(7q), and an add(17q) in association with trisomy 13. In addition, immunophenotypic analysis revealed the coexpression of the CD33 and CD7 antigens on common ALL blasts, in accordance with other reported cases that displayed a predominant biphenotypic leukemia profile. The patient failed to obtain a second remission and died soon after due to infective complications. This report indicates that trisomy 13 can be found also in B-lineage ALL and underlines that this cytogenetic abnormality may identify a subgroup of male patients with clonal evolution potential and an adverse clinical outcome.