The use of new nuclei probes in fluorescent in situ hybridization (FISH) at diagnosis and during follow up has recently allowed the detection of a deletion of the 5'abl region on the derivative chromosome 9 among some CML patients. This deletion seems to be a powerful and independent prognostic factor. The aim of our study was not only to estimate the frequency of the deletion of the 5'abl region among chronic myeloid leukemia (CML) patients with bcr-abl fusion gene, but also, to assess whether this deletion is concomitant with the formation of the Philadelphia (Ph) chromosome or represents a sign for progression of the disease, and finally to evaluate the prognostic implications of this abnormality. One hundred and twelve patients were analysed using FISH with LSI bcr-abl dual ES color probes, at the moment of the diagnosis when possible or, if not, on a sample with a strong rate of Ph+ metaphases evaluated by conventional cytogenetics. When the deletion was highlighted in a patient, we performed an hybridization on all the samples available during the follow-up. The deletion of the 5' region of the gene abl was detected among 9 patients. When the deletion was found in a patient, it was present in all the Ph+ metaphases and nuclei and in all the samples studied at diagnosis and during follow up. In these patients, we never identified cells carrying the Ph chromosome translocation without the deletion. None of the patients with the deletion had a major cytogenetic response to treatment with interferon. The deletion of the 5'abl region on der(9), present in approximately 9% of the CML, takes place at the same time as the formation of the Ph chromosome translocation and seems of worse prognosis. The detection of this deletion could thus constitute an argument to start STI treatment in first intent for these patients.