A retrospective chart review of 427 eyes diagnosed with unilateral retinoblastoma was performed to determine which eyes, which patients, and when new intraocular tumours would develop after treatment. Mean follow up was 8.16 years. Twenty five (6%) of 427 unilateral retinoblastoma patients developed new intraocular tumours after treatment. Five (1%) unilateral patients who were previously treated with enucleation developed new tumours (in the fellow eye). Fifteen (24%) unilateral patients who were previously treated with external beam radiation developed new tumours (equally in either eye). New tumours did not develop in the macula of either eye. The relative risk of developing new intraocular tumours after treatment was 16% in patients diagnosed before 1 year old and 2.2% for patients diagnosed after 1 year old (p < 0.001). The mean time to onset for the development of new tumours after treatment was 0.74 years; no new tumours appeared after 7.5 years of age. Those patients who are diagnosed with unilateral retinoblastoma in the first 6 months of life and have a family history of the disease are at greatest risk of developing new intraocular tumours.