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Tumour-associated epilepsy accounts for a quarter of paediatric patients undergoing epilepsy surgery with the vast majority achieving long-term seizure and drug freedom. We report the case of an eight-year-old patient who presented with developmental delay and overgrowth, followed by temporal lobe seizures that were attributed to a mesio-temporal brain tumour, and who was eventually treated with epilepsy surgery. Histopathology revealed a diffuse astrocytoma but its gross total resection surprisingly failed to control the temporal lobe seizures. Genetic testing identified a de novo pathogenic variant in the NSD1 gene, thus establishing the diagnosis of Sotos syndrome. Sotos syndrome is a rare overgrowth syndrome with an increased incidence of malignancy, including the very rare occurrence of brain tumours. Seizures are frequent in patients with Sotos syndrome, often occurring with temporal lobe semiology and ictal EEG patterns in the absence of a brain lesion, and usually responding to anti-seizure medication. Our case highlights Sotos syndrome as a rare but important pitfall in the presurgical workup of temporal lobe epilepsy that should be considered particularly in MRI-negative cases but also in the presence of a focal lesion that does not fully explain the clinical picture. Most importantly, our observations underline the value of thorough presurgical diagnostics including genetic testing, even in apparently straightforward cases of lesional epilepsy, to rule out an underlying genetic aetiology that may not be treated by surgery. Finally, our findings emphasize the need to re-evaluate our less successful epilepsy surgery cases and offer informed counselling and prognostication.