Lymphocytes from 3 children with a form of juvenile amaurotic idiocy (characterized by retinal blindness, progressive dementia, and extrapyramidal motor disturbance) were studied by electron microscopy. Numerous fingerprint profiles (FP) were found in vacuolated lymphocytes from all 3 patients and an asymptomatic younger sibling of 1 patient who subsequently became symptomatic. We propose that the combination of this clinical picture and vacuolated lymphocytes with FP is sufficiently distinctive for clinical and research purposes until the biochemical defect is discovered. Wider utilization of ultrastructural study of lymphocytes should increase the number of children diagnosed and allow detection of asymptomatic patients.