Börjeson-Forssman-Lehmann syndrome (BFLS) is a rare X-linked mental retardation syndrome that is caused by germline mutations in PHF6. We describe a 9-year-old male with BFLS, who developed T-cell acute lymphoblastic leukemia (T-ALL). The PHF6 gene is located on the X chromosome and encodes a protein with two PHD-type zinc finger domains and four nuclear localization sequences. Previously, overexpression of Phf6 was observed in murine T-cell lymphomas. Our observation indicates that BFLS may represent a cancer predisposition syndrome and that mutations of PHF6 contribute to T-ALL. Pediatr Blood Cancer.