A case of primary mucinous carcinoid of the ovary observed in a 52-year-old woman is reported after some remarks on the various histogenetic theories. The observation of this extremely rare cancer was fortuitous during surgery carried out for benign pathology in the contralateral organ. Careful postoperative controls absolutely excluded the possibility of a metastatic tumour. The patient was treated by combining surgery with chemotherapy and eight months after operation is apparently free of cancer.