The authors report an extremely rare case of adenoid cystic carcinoma of the prostate. No satisfactory clinical and pathological classification exists for this tumour, creating particular difficulties for therapeutic decisions and prognosis. Descriptions of cases with excellent survival rates are reported in the literature, to the extent that this is sometimes regarded as a low-malignancy tumour, but other reports also exist of massive diffusion of the pathology with fatal consequences. Advances in the knowledge of this tumour have enabled a number of earlier pathogenetic hypotheses to be ruled out, namely its possible derivation from ectopic salivary cells, ectopic periurethral glands or metaplastic urethral mucosa, but the origin of this carcinoma is still not certain. It is also difficult to differentiate this form from the typical adenocarcinoma and to make a prognosis for survival. In the case reported here, the final diagnosis was made on the lymph node biopsy obtained during surgery, given that a preoperative biopsy was not feasible owing to the scarcity of material available. The patient received standard hormonal therapy for prostate carcinoma.