Approximately 30% of the cases of retinoblastoma (RB), the childhood eye cancer, are inherited and are manifested by unilateral or bilateral tumor. In sporadic tumors, accounting for 70% of cases, only one eye is affected. RB has three histological features: undifferentiated anaplastic cells, retinoblast pattern, and differentiated pattern characterized by Flexner Wintersteiner rosettes (FWR). Currently, results concerning phosphoprotein RB (pRB) expression in RB tumors are contradictory. In this study we detected pRB immunohistochemically in 10 tumors from bilateral or unilateral RBs, which did not show gross chromosomal alterations in cytogenetic studies. Interestingly, pRB was undetectable in only one tumor where we found distinct histological features. Our results suggest that pRB immunopositivity may be common in these tumors. However, it does not rule out the possibility that pRB is functionally inactive in some cases. This may be due to the protein being present in phosphorylated form or being altered by point mutations not affecting its expression. Another possibility is that mechanisms other than RB1 gene changes may lead to retinoblastoma because not all cases of retinoblastoma show gene alterations. Together these findings may be useful in understanding the molecular mechanisms associated with this type of pediatric tumor. © 2007 Elsevier GmbH. All rights reserved.