We report on a girl with a large interstitial deletion of the long arm of chromosome 21 and with mild mental retardation, congenital hypothyroidism, and hyperopia. The deletion [del(21)(q11.1-q22.1)] extends molecularly from marker D21S215 to D21S213. The distal breakpoint is not clearly defined but is situated between markers D21S213 and IFNAR. This patient has the largest deletion of chromosome 21 known without having severe mental retardation or malformations. The deletion does not involve the "Down syndrome chromosome" region, the region of chromosome 21 which in trisomy causes most of the manifestations of Down syndrome. Apparently, the proximal part of the long arm of chromosome 21 does not include genes that are responsible for severe clinical effects in the event of either deletion or duplication, since several reported patients with either trisomy or deletion of this region have mild phenotypic abnormalities. Congenital hypothyroidism is much more common in Down syndrome than in the average population. Thus, the congenital hypothyroidism of the present patient might indicate that there is one or several genes on the proximal part of chromosome 21, which might be of importance for the thyroid function.