This study presents previously unreported ultrastructural support for a model for the incidence of retinoblastoma based upon a two-step mutation theory. Ostensibly uninvolved retina showed rod outer segment atrophy and cone outer segment retention correlating with electroretinography, and obliteration of synaptic development within the outer plexiform layer of the retina. The retinoblastoma obtained at age 9 days demonstrated incipient photoreceptor differentiation within the rosette components and minimal calcification of necrotic regions. A heritable interstitial deletion of the long arm of chromosome 13 is shown to be the basis for this child's congenital retinoblastoma.