The patient presented with the typical features of Down syndrome; hypotonia, brachycephaly, flattened occiput, bilateral prominent medical epicanthic folds, flat nasal bridge, protruding tongue, low-set dysplastic ears, short broad hands, bilateral clinodactyly and simian crease. The karyotype of this child was originally reported as normal. High-resolution chromosomes revealed extra material on the long arm of chromosome 18. The mother's karyotype showed a reciprocal translocation between the long arm of 18 and the long arm of 21 at band q23 and q22.1, respectively. FISH performed separately with two different 21q cosmid probes gave two signals on the mother's metaphases and three signals on the proband. These findings confirmed that the proband is trisomic for the long arm of chromosome 21 at loci D21S65 and D21S19.