A severely mentally retarded man displayed the following main symptoms: short stature, microcephaly, antimongoloid slant of palpebral fissures, big ears with hyperplastic helices, imperfect dental enamel, short and webbed neck, short arms, short hands, brachymetaphalangy, short second fingers, broad thumbs, short metatarsal bones, and unusually big first toes. It seems almost certain that the syndrome was caused by a chromosome deletion involving about half of 12p which was present in all of the lymphocytes examined.