Haematological problems are commonly associated with use of beta-interferon in multiple sclerosis (MS) patients. However these problems are usually benign and are reversible when the drug is stopped. We describe two patients, with the diagnosis of MS, who developed leucocytosis. Cytogenetic studies showed the presence of the Philadelphia chromosome in all the cells analysed, confirming the diagnosis of chronic myeloid leukaemia (CML). The first patient currently on imatimib mesylate and interferon beta-1a is asymptomatic. For the second patient, interferon beta-1a was withdrawn after suspicion of CML.