Acute basophilic leukaemia is usually characterized by a very rapid clinical course, hyperhistaminemia, resistance to antineoplastic therapy and early death due to complications related to disease. This entity is a rare condition, accounting for less than two percent of all haematopoietic malignancies. Most of the case reports are basophilic blast crisis in patients with a previous lympho or myeloproliferative disorder. A 62-year-old woman who was diagnosed as Philadelphia positive chronic myelogenous leukaemia after four years of evolution developed a basophilic blast crisis, whose characteristics are reported. Accompanying this transformation there was also a cytogenetic change. Despite chemotherapy the patient died of disease progression.