A 48-year-old-man in the first chronic phase of chronic myeloid leukaemia developed a central nervous system complication on day +57 after HLA-identical peripheral blood progenitor cell (PBPC) transplantation. The clinical picture evolved to a reversible pseudobulbar palsy requiring mechanical ventilation. MRI examination disclosed lesions typical of central and extrapontine myelinolysis (CEPM), which disappeared on a repeat examination 20 days later. The patient had received cyclosporine A (CsA) as GVHD prophylaxis and severe hyponatremia was detected 7 days after the first neurological sign. CEPM has been described in alcohol-induced liver disease, following rapidly corrected hyponatremia and associated with CsA in orthotopic liver transplantation. This is the first reported case of CEPM in PBPC transplantation, and CsA seems to have played a role in the development of this very serious complication.