We report the case of a 70-year-old man with symptoms of urinary obstruction and haematuria, with histological diagnosis of primary signet-ring cell carcinoma of the prostate. Almost 90% of the tumour cells contained characteristic intracytoplasmic vacuoles that positively stained with diastase-digested PAS, Alcian blue and mucicarmine. The positive immunostaining for PSA and PSAP confirmed the prostatic origin of the tumour. Although the patient received hormonal therapy, the disease progressed and the patient died 11 months after surgery.