Retinoblastoma (Rb) occurs in two different genetic forms, hereditary and nonhereditary. Patients with nonhereditary Rb have unilateral, unifocal tumors, and those with the hereditary form usually have bilateral, multiple tumors. However, about 15% of patients with unilateral disease actually have multifocal tumors due to the hereditary form of Rb. Distinction between unilateral hereditary and sporadic Rb is very important both to the patient, who is at risk of later development of other malignancies, and to any future siblings and offspring. With only clinical data, the discrimination may be difficult. The cytogenetic detection of multiple unrelated clones in the tumor is very helpful for differential diagnosis because this finding strongly suggests the presence of a prezygotic germ cell mutation, which is heritable. A 23-month-old Chinese boy was found to have a single retinoblastoma at the right eye. Cytogenetic study of the tumor cells revealed three abnormal clones. Two of them were related, but the third one had completely different markers, suggesting a hereditary, multifocal origin of the tumor. None of the three clones showed an abnormality of 13q, which is present in about 21% of all Rb patients.