Introduction: Individuals who are exposed to cytotoxic agents are at risk of developing therapyrelated myeloid neoplasms (t-MN). Cytogenetic findings of a neoplasm play an important role in stratifying patients into different risk groups and thus predict the response to treatment and overall survival. Case report: A 59-year-old man was diagnosed with acute promyelocytic leukaemia. Following this, he underwent all-trans retinoic acid (ATRA) based chemotherapy and achieved remission. Four years later, the disease relapsed and he was given idarubicin, mitoxantrone and ATRA followed by maintenance chemotherapy (ATRA, mercaptopurine and methotrexate). He achieved a second remission for the next 11 years. During a follow-up later, his full blood picture showed leucocytosis, anaemia and leucoerythroblastic picture. Bone marrow examination showed hypercellular marrow with trilineage dysplasia, 3% blasts but no abnormal promyelocyte. Fluorescence in-situ hybridisation (FISH) study of the PML/RARA gene was negative. Karyotyping result revealed complex abnormalities and monosomal karyotype (MK). A diagnosis of therapy-related myelodysplastic syndrome/myeloproliferative neoplasm with unfavourable karyotypes and MK was made. The disease progressed rapidly and transformed into therapy-related acute myeloid leukaemia in less than four months, complicated with severe pneumonia. Despite aggressive treatment with antibiotics and chemotherapy, the patient succumbed to the illness two weeks after the diagnosis. Discussion and Conclusion: Diagnosis of t-MN should be suspected in patients with a history of receiving cytotoxic agents. Karyotyping analysis is crucial for risk stratification as MK in addition to complex aberrant karyotypes predicts unfavourable outcome. Further studies are required to address the optimal management for patients with t-MN.