The gene locus of esterase D is closely linked to that coding for retinoblastoma. When the occurrence of retinoblastoma is based on a chromosome deletion, red cell esterase D might be a potential tumor marker for diagnosing retinoblastoma. This diagnostic utility was tested by measuring total esterase D and differentiating esterase D to its different phenotypes in red cells of patients with bilateral retinoblastoma, unilateral retinoblastoma having a positive family history and unilateral sporadic retinoblastoma. These results are compared with the findings within a group of first degree relatives of these patients and of a reference group of apparently healthy controls. A poor sensitivity and a low positive predictive value were found. So, the conclusion might be drawn that screening of all retinoblastoma patients for esterase D in order to get insight in the weighting of risk of retinoblastoma is very much open to question and due to high cost efficiency ratio might not be recommended.