Clonal rearrangements of immunoglobulin (Ig) and T cell receptor (TCR) genes have been demonstrated in malignant lymphoid tumors of B and T cell origin. In Philadelphia chromosome (Ph1) positive chronic myeloid leukemia (CML) the bcr and c-abl genes are reorganized and a new transcript, composed of both genes is expressed. Immunoglobulin gene rearrangements were also detected in lymphoid blast crisis but not in myeloid blast crisis of CML. We analyzed in Southern blot experiments whether Ig and TCR rearrangements could also occur in the chronic or accelerated phase of the disease. Our results indicate that immunoglobulin but not TCR delta or TCR gamma gene rearrangements also occurred in some patients with CML in chronic and accelerated phase but not in myeloid blast crisis, together with rearrangements of the bcr gene.