A series of 39 retinoblastomas was studied using light microscopy and immunohistochemistry for localization of neurone-specific enolase and glial fibrillary acidic protein. Thirty-eight retinoblastomas (97.4%) occurred in children less than 6 years of age: one was in a 61-year-old man (2.6%); the mean age, excluding the 61-year-old, was 32.1 months. Unilateral tumours were found in 74.4% of patients and bilateral tumours in 25.6%; bilateral lesions occurred in a younger age group, mean age of 21.7 vs 35.9 months. The male to female ratio was 1.4:1. Morphologically, arrangement of tumour cells into trabeculae was noted in 69.2% of retinoblastomas and glomerulus-like structures in 43.6%. These two patterns of cellular arrangement were encountered where the choroid and sclera were invaded by neoplastic cells. Homer Wright rosettes were observed in all tumours. Flexner-Winstersteiner rosettes in 33.3% and fleurettes in 2.6%. Differentiation into neoplastic neurones as well as in the form of Flexner-Winstersteiner rosettes and fleurettes was noted in 46.1%, neoplastic ependymal cells in 100.0%, astrocytes in 58.9%, oligodendrocytes in 23.1%, and glioblastomas in 2.6%. These data suggest that retinoblastoma is a primitive stem cell neuroectodermal tumour with the capacity for differentiation in both neuronal and neuroglial directions.