In the family described here, the grandmother, father, and 3 of his 9 children had an apparently abnormal chromosome of group 21–22. The abnormality was similar in morphology in all the affected subjects and was found in metaphases from cultures of leukocytes, and skin and bone marrow. The short arm of the abnormal chromosome seemed longer than normal; it was not possible to ascertain whether the satellite itself or the short arm, or both, were enlarged. Of the 5 subjects affected, 4 were clinically normal. The fifth, a girl, had severe abnormalities of the central nervous system. One of her brothers, who was cytogenetically normal, had a similar syndrome of unknown origin. From these data, and from those published by others, it was postulated that an apparently enlarged satellite region is compatible with normal development. Such an abnormality does not necessarily imply an association with developmental disorder in the offspring of carrier subject