The authors have analysed the esterase-D levels in 500 retinoblastoma patients of whom 15 showed red cell enzyme activities of approximately 50% that of normal controls. Chromosome analysis of these 15 patients confirmed the presence of a deletion involving region 13q14 in all cases. Seven of the 15 cases had not previously been diagnosed and all of these showed sub-band deletions within 13q14. None of these seven patients were mentally retarded although the remaining eight who showed larger chromosome deletions demonstrated the full spectrum of psychomotor abnormalities associated with 13q deletions. Two other mentally retarded retinoblastoma patients with normal esterase-D activity showed no karyotypic abnormality, demonstrating that mental retardation cannot be taken to indicate a chromosome deletion in all cases. Eight of the 15 deletion cases were only unilaterally affected. The data presented in this article suggest that esterase-D quantitation could provide the primary means of detection of chromosome deletions in retinoblastoma patients.