Clonal chromosomal aberrations are reported in about 25% of the patients with hairy cell leukemia (HCL). No consistent cytogenetic abnormality has been described in HCL; most of the chromosomal changes found have been deletions and inversions, with the rare occurrence of translocations. While most of the chromosomal aberrations in HCL are common to the ones found in B cell chronic lymphocytic leukemia and other B cell lymphoproliferative disorders, there are also certain chromosomal changes that are not found in other B cell lymphoproliferative disorders. We present here a 63-year-old male patient with hairy cell leukemia with the clonal del(17)(q25), which has nor previously been reported in HCL. (C) Elsevier Science Inc., 1998.