Radiogenic or non-radiogenic osteosarcomas were found in approximately 15% of the observed bilateral retinoblastoma cases, and only exceptionally in the unilateral cases. Such osteosarcomas are also seen in subjects without retinoblastoma, but belonging to affected families. Late second malignant extraocular tumours were only observed when we were dealing with a germinal mutation, that is in hereditary retinoblastoma. They are due to a pleiotropic effect of a single gene situated on the chromosome D13 (ql4), or to two different but very close genes.