Retinoblastoma patients have a strongly increased risk of second malignancies, and survivors with a third or subsequent malignancy are increasingly observed. However, it has not been examined whether survivors who developed a second malignancy have a greater risk of a subsequent malignancy. On the basis of the Dutch retinoblastoma registry, the risk of a third malignancy was compared with cancer risk in the Dutch population. Cox model analysis with a time-dependent covariate was used to compare the subsequent malignancy risk and survival among patients with and without a second malignancy. Risk of a third malignancy was increased 8-fold compared with the general population. The hazard ratio (HR) of a third malignancy after a second malignancy was more than 7-fold increased compared to the risk of a second malignancy after retinoblastoma. Radiotherapy increased the risk 3-fold. A third malignancy was associated with worse survival compared with survival of patients only diagnosed with a second malignancy (HR = 5.0). Survivors of retinoblastoma who already developed a second primary malignancy have an even higher risk of subsequent primary malignancies than retinoblastoma survivors without a second malignancy. Treating physicians and patients should be aware of this higher risk. © 2010 Elsevier Ltd. All rights reserved.