Chromosomal analysis of a 4-months-old female child with rotatory spontaneous nystagmus, defective vision (blindness?) and advanced motor and mental retardation of development shows an E-trisomy-mosaic with accompanying translocation C/E. The 2 1/2-years old brother of this child shows the same clinical symptoms, and cytogenetical investigation leads to the suspicion of the presence of the same chromosomal aberration. The karyotypes of the parents are normal. The cytogenetical findings and their relationship to the clinical condition are discussed. © 1970 Springer-Verlag.