We describe a 3-year-old boy and his 2 maternal uncles with moderate to severe mental retardation, short stature, mild obesity, hypogonadism, a low total finger ridge count, and a distinctive face characterized by bitemporal narrowness, almond-shaped palperbral fissures, depressed nasal bridge, anteverted nares, short and inverted-V-shaped upper lip, and macrostomia. Two other males in this family who had similar facial anomalies and developmental delay died in early infancy and midchildhood. This apparently new disorder is reminiscent of, but distinct from, the Prader-Willi syndrome, and is likely inherited as an X-linked recessive trait. Preliminary studies with DNA probes are consistent with an X-linked locus and permit exclusion of distal Xp and Xq regions as the site of this mutation.