Two cases of uterine carcinosarcoma developing after long-term tamoxifen (TAM) treatment are presented. The patients, 67 and 72 years old, were treated with TAM for 6 and 7 years, continuously. They both developed an heterologous malignant mixed Mullerian tumor (mmMt). At laparotomy, an advanced stage of disease was found with peritoneal spread. In spite of the surgical and the postoperative treatment, they both died of disease, 3 and 10 months later. There are only 10, well documented, similar cases reported. Another 7 were identified in series of uterine malignancies developing after TAM treatment. Considerable evidence suggests that mmMt represents an epithelial cancer with sarcomatous dedifferentiation. Prolonged (> 5 years) TAM treatment may represent a causative factor in the development of this highly lethal disease (80% of the reported patients had a dismal prognosis). Large uterine polyps with special histological features, may represent an intermediate step in the tumor formation. Close follow-up of the patients is warranted.