Cystic neoplasms arising from the prostate are rare, and stromal tumours of uncertain malignant potential and the spectrum of cystic epithelial tumours of the prostate are the major differential diagnoses of a cystic prostatic neoplasm. We report a case of a stromal tumour of uncertain malignant potential, which showed a multilocular cystic mass with some solid components. The solid component of the tumour did not show substantial diffusion restriction and uptake of 18F-FDG-PET, and this could be the critical finding suggesting a stromal tumour of uncertain malignant potential rather than a malignant cystic epithelial tumour.