Intra-abdominal desmoplastic small round cell tumours are rare aggressive tumours of mesothelial origin with less than 60 cases reported in the literature. They are difficult to treat and the role of 18F-fludeoxyglucose positron emission tomography (PET)/CT scan in their management has not been established. A 41-year-old male presented with a 21-cm desmoplastic small round cell tumour and was managed with radiotherapy, surgery and chemotherapy, with each treatment monitored and guided by 18F-fludeoxyglucose PET/CT scan. We present the imaging findings of the serial PET/CT scans of this patient and their impact on management.