A series of 16 cases of retinoblastoma diagnosed by fine-needle aspiration biopsy (FNAB) and confirmed by histologic examination is reviewed, and the salient cytomorphologic features are described. Two types of cells were encountered in the aspiration smears; type I cells were undifferentiated while type II cells showed more differentiation and frequently revealed cytoplasmic processes that are probably indicative of early photoreceptor differentiation. Flexner-Wintersteiner rosettes characteristic of retinoblastoma were found in 10 of 16 cases. These findings were further correlated with ultrastructural examination of the tumors in nine cases. It is concluded that the presence of rosettes and type II cells with cytoplasmic processes are the two features that are most helpful in the FNAB diagnosis of retinoblastoma.