The paper describes a case of Philadelphia (Ph) positive acute lymphoblastic leukaemia (ALL) presenting with high white cell count and central nervous system involvement. Immunophenotypically the case was characterized as common ALL. The t(9;22) abnormality corresponded to a rearrangement within the breakpoint cluster region gene, while antigen receptor gene studies showed multiple rearrangements of the immunoglobulin heavy chain gene (IGH) concomitant with a single rearrangement of the T cell receptor beta chain gene (TCR beta). We speculate that this case represents the neoplastic transformation of a stem cell, the Ph abnormality being involved in the early steps of transformation. It is conceivable that the IGH but not the TCR beta gene was accessible to recombination within the malignant clone, thus generating the multiple rearrangements observed. If this is the case, these findings would appear to be compatible with the hypothesis that antigen receptor gene rearrangements may be partly dependent on the accessibility of the corresponding genetic loci.