Retinoblastoma(RB) patients have a high risk to develop second malignant neoplasm (SMN). Diencephalic tumors(DT) are rare in paediatric age. A retrospective review was performed over 21 years period. Out of 169 RB patients, 3 presented a DT. Two patients presented a Rb1 germline mutation and none received radiotherapy. DT in previously treated RB patients seems a peculiar SMN. However, considering the site, the short time interval from RB and the absence of radiotherapy, an alternative pathogenic mechanism could be supposed. The same embryological origin of the retina and the diencephalon should be considered and biological studies are needed.