Treatment-related chronic myeloid leukaemia (CML) is a less commonly recognised entity. It differs from treatment-related acute leukaemias in frequency, clinical course, and prognosis. Previously two cases of CML have been described following treatment of myasthenia gravis associated with thymoma treated by thymectomy. We report a 25-yr-old man with myasthenia gravis without thymoma who developed CML, 68 months after thymectomy. Cytogenetic study showed translocation (9,22)(q34;q11) with 10% showing double Ph chromosome. Reverse-transcriptase polymerase chain reaction (RT-PCR) of peripheral blood demonstrated the presence of p210(BCR/ABL) with b3a2 transcripts. He was treated with hydroxyurea, and still remained in the chronic phase during the last 6 months of follow up. His myasthenic symptoms remained stable.