Three mentally retarded non-mongolid children with very similar clinical signs and an extra G-like chromosome are presented. In one of these patients, Case I, an attempt to trace the origin of the extra chromosome has been made by QM fluorescence analysis and autoradiography. Visually the QM pattern of the extra chromosome differs from that of chromosomes 21 and 22 by showing a diffuse fluorescence of intermediate intensity in the long arm, not unlike that found in the proximal part of the long arm of chromosome no. 13. However, whether these are sufficiently similar to indicate homologous regions cannot be decided. The autoradiographic pattern is not decisive as to the origin of the extra chromosome; it shows early labelling, as do no. 22 and the short arm and proximal part of the long arm of no. 13. Case II, a sibling of Case I, died in 1962 at the age of 6 years (Gustavson et al. 1962). In Case III the autoradiographic pattern of the five G-group chromosomes is similar to that found in Case I. The clinical and cytogenetic features of these 3 patients are compared with those of 9 other previously reported patients with a similar phenotype and an extra small acrocentric chromosome. These 12 patients seem to constitute a specific clinical syndrome.