Leiomyomatosis peritonealis disseminata (LPD) is a rare smooth muscle tumour characterized by multiple small nodules on the omentum and peritoneal surface, composed of benign smooth muscle cells with minimal mitotic activity, frequently admixed with decidual cells. The possible pathogenetic mechanisms include hormonal dysfunction, differentiation of subperitoneal mesenchymal stem cells, myofibroblastic metaplasia and genetic and iatrogenic causes (resection of myomas during laparoscopic surgery). Diagnosis is easily made on biopsy specimens. Reduction of oestrogen exposure, surgical castration or gonadotrophin releasing hormone agonists are generally sufficient to cause regression of LPD. We report a case of an asymptomatic 36-year-old pregnant woman with long-term use of oral contraceptives, and previous myomectomy, who had a mass of uncertain origin which was, histopathologically, diagnosed as leiomyomatosis peritonealis diffusa with foci of ectopic decidua. Ectopic decidua was also present in a pelvic lymph node. To the best of our knowledge, this is the first case of LPD containing foci of ectopic decidua in a pregnant woman with a past history of myomectomy and use of oral contraception for three years; ectopic decidua was also detected in a pelvic lymph node.