Tumour lysis syndrome is a catastrophic complica­lion of treatment of certain neoplasms. It is usually seen with the tumours that have high growth fractions, increased bulk and extreme sensitivity to cytotoxic therapy. Most commonly it occurs in association with hematologic malignancies such as lymphomas and leukaemias. Rarely it has been observed with solid tumours like small cell lung cancer, seminoma and carcinoma of the breast. The syndrome develops due to massive cell lysis occurring within a few hours to a few days of starting the cytotoxic therapy. It is characterized by hyperuricemia, hyperkalemia, hyperphosphatemia and hypocalcaemia. Acute renal failure, cardiac arrhyth­mias and sudden death may occur. Tumour lysis syndrome has hitherto not been reported as a complica­lion of radiotherapy only. We recently observed a patient with a diagnosis of chronic myeloid leukaemia who received splenic irradiation for massive splenomegaly and developed full-blown tumour lysis syndrome. Pathogenesis and clinical implications are discussed.