Purpose of Investigation: The authors aimed to assess the hypothesis that leiomyomatosis peritonealis disseminata (LPD) in a postmenopausal woman had autocrine estrogen secretion abilities. Materials and Methods: A 73-year-old woman presented with a bulky tumour in the peritoneal cavity. The pathological diagnosis was LPD or sarcoma of unknown origin. Medroxyprogesterone acetate (MPA) and GnRH analogue therapies were effective, but a tumour mass was confirmed with rapid growth immediately after the cessation of treatment and led to her death from intra-tumour haemorrhage. The authors evaluated the mechanism of tumour using immunostaining for aromatase cytochrome P450. Results: The tumour showed high expression of aromatase. The immunostaining patterns of estrogen receptor (ER), progesterone receptor (PgR), and aromatase were similar to those of a premenopausal leiomyoma. Conclusion: It is likely that the tumour had the autocrine ability to secrete estrogen and that tumour growth resumed because of a flare-up caused by the downregulation of hormonal therapies.