Osteosarcoma is the most common second neoplasm in patients with retinoblastoma. The risk of occurrence of second neoplasm after retinoblastoma increases after radiotherapy and chemotherapy. A case is presented of an 11-year-old girl who had painless swelling on the left side of her face for 8 days. The patient was asymptomatic before the onset of the swelling. Biopsy of the lesion revealed tumor cells forming trabeculae of osteoid and woven bone interspersed with myxoid and cartilaginous areas. The tumor cells showed pleomorphism and hyperchromatism with increased and abnormal mitotic figures consistent with the diagnosis of osteosarcoma. Further investigations revealed no history of retinoblastoma in the family. (This case was considered hereditary, however, because of the occurence of the second neoplasm.) To the best of our knowledge, this is the first case of mandibular osteosarcoma occurring after unilateral retinoblastoma treated with chemotherapy. Schlagwörter: chemotherapy, mandible, osteosarcoma, RB 1 gene, retinoblastoma, second neoplasm