We present a Ph-positive chronic myeloid leukaemia patient who lost a complete cytogenetic response (CCR) of 23 months duration at the time of detection of a deletion, not previously observed, of chromosomes 9 and 22 sequences flanking the translocation breakpoint on the derivate 9 chromosome. To our knowledge, this is the first case in which a deletion at the t(9;22) breakpoint has arisen as a secondary genetic alteration produced after formation of the t(9;22) translocation. It remains to be determined whether this genetic abnormality has the same prognostic importance as when observed at diagnosis.