Background: Imatinib mesylate, a competitive inhibitor of tyrosine kinase is considered the treatment of choice for patients at all stages of chronic myeloid leukaemia (CML). In the majority of patients, the drug is well tolerated. However, a significant number of haematological complications including anaemia, thrombocytopenia, neutropenia, Pancytopenia and autoimmune haemolytic anaemia have been reported. Materials and Patients: This study prospectively investigated the spectrum and severity of haematologic anomalies associated with the use of imatinib in a population of Nigerian CML patients. Consenting Philadelphia chromosome and/or BCR-ABL1 positive CML patients seen at the Department of Haematology and Blood Transfusion (OAUTHC), Ile-Ife, in chronic and accelerated phases and managed with imatinib were recruited. They were commenced on 400-600 mg of imatinib. Full blood count was obtained before, and then at three and six months after commencing imatinib, using an auto-analyser (Sysmex PocH-100i) to assess the effect of the drug on haemoglobin concentration, platelet count and absolute neutrophil count (ANC). Twelve patients who developed transfusion-dependent, imatinib-related anaemia had bone marrow aspiration.