Primary mucinous cystic neoplasms are rare tumours of the kidney, with a very few case reports in the literature. They arise from metaplasia of renal pelvic urothelium. We describe here a 45-year-old male who presented with pain in the abdomen and a lump in the left iliac fossa for two months. Ultrasound and CT scan showed a large, complex, heterogenous mass in the central abdomen and left iliac fossa, suggesting the possibility of dermoid cyst. Excision of the mass showed an enlarged multicystic kidney fi lled with mucin, destruction of renal parenchyma and a small viable area of grey white tumour. Histopathology revealed a peripherally located mucinous cystadenocarcinoma arising in the background of chronic pyelonephritis and mucinous metaplasia. We report this case for the rarity of the lesion and the associated clinical and radiological diagnostic dilemma.