The hereditary form is usually bilateral and multifocal while the non-hereditary form is generally unilateral and unifocal. However, fifteen percent of unilateral cases are hereditary. Bilateral cases often present earlier than unilateral cases.
Unilateral tumors are often large by the time they are discovered.
The average age at diagnosis is 15 months for bilateral cases compared to 25 months for unilateral cases. It is unusual for a child to present with a retinoblastoma after two years of age. Occasionally, the tumor is discovered at birth, during adolescence, or even in adulthood. On examination the tumor appears as a white mass, sometimes small and relatively flat, sometimes large and protuberant. It may appear nodular.