Survivors of hereditary retinoblastoma have increased risk of subsequent primary malignancies due to RB1 mutation. We report uterine leiomyosarcoma (LMS) in a hereditary retinoblastoma survivor. She had follow-up for leiomyomas, with pelvic MRI showing typical leiomyomas two years prior. She presented with abdominal distention, and MRI revealed a massive tumor with LMS characteristics where a leiomyoma was previously observed. Chest CT showed a nodule suspicious for metastasis in the left lung. Total hysterectomy with bilateral salpingo-oophorectomy and partial lung resection was performed. Pathology confirmed LMS with pulmonary metastasis. Immunostaining showed complete RB1 loss in tumor cells. LMS was suspected to have arisen near a pre-existing leiomyoma or resulted from its malignant transformation. Continuous follow-up is necessary in hereditary retinoblastoma survivors.