Two cases of del(13)-retinoblastoma are reported. Case 1, a 13-month-old male, was monosomic due to the malsegregation of the maternal ins(20;13)(p12;q1307q14.3). The patients's sister was trisomic for 13q1307q14.3 with no evident phenotypic effect. Case 2 was a 20-month-old female with a denovo del(13)(q1303q14.3). In both instances esterase D activity showed a remarkable gene-dosage effect in monosomy, disomy, and trisomy, thus confirming the assignment of the gene locus to 13q14, and more precisely to the proximal half of this band. In all instances, the ESTD phenotypes were 1-1. It is suggested that esterase D activity should become an important diagnostic criteria for the various etiological forms of retinoblastoma.