The transition from the benign retinal tumor retinoma to its malignant counterpart retinoblastoma is accompanied by the loss of expression of the p75NTR neurotrophin receptor. This change in expression is mimicked in the TAg-RB murine model of retinoblastoma, where early tumors retain expression of p75NTR and advanced tumors lack it. We sought to determine the functional effect on tumor development of absence of p75NTR from the onset of TAg-RB tumor initiation. TAg-RB mice were crossed with either p75NTR exon 3 (E3KO) or exon 4 knockout (E4KO) mice to produce TAg-RB offspring that lacked one or both normal p75NTR alleles. The average tumor area per eye as a percentage of retinal area was measured. TAg-RB/E3KO (TAg-RBE3KO) and heterozygous mice showed no significant difference in tumor area compared to the TAg-RB control mice at any time point studied. However, TAg-RB/E4KO (TAg-RBE4KO) and heterozygous mice displayed a significantly larger tumor area than the TAg-RB control mice. Furthermore, adenoviral-mediated expression of p75NTR in a p75NTR-deficient human retinoblastoma cell line resulted in increased apoptosis. Our results confirm that p75NTR suppresses progression of both human and TAg-RB murine retinoblastoma, and holds promise as a target for future therapy of the disease. © 2008 Wiley-Liss, Inc.