We report on a new X-linked recessive syndrome in 2 unrelated families, consisting of pre- and postnatal growth excess, typical facial phenotype allowing diagnosis at birth, and usually normal physical and intellectual development. The minor anomalies seen at birth include a "coarse" face with wide nasal bridge, short nose with upturned nasal tip, wide open mouth, thick lips, midline depression of the lower lip, enlarged tongue, highly arched palate, large maxilla and jaw, and a short broad neck. Voice is hoarse and affected individuals have a plump, stocky body with pectus excavatum, thoracic scoliosis, hepatosplenomegaly, umbilical and/or inguinal hernias, broad short hands and feet, and in some cases preauricular dimples, abnormal ears, postaxial hexadactyly, hypoplastic index finger nails, and abnormal dermatoglyphics. Early postnatal death is frequent and pathogenetically unexplained. During infancy and childhood the leading manifestations are the overgrowth (greater than 97th centile), striking facial appearance, hypodontia and/or malposition of teeth, genua valga, hypoplastic calf muscles, and clumsiness. Adolescent and adult patients have well proportioned "gigantism" of athletic build (192-210 cm), large "coarse" face, and a deep voice. General intellectual and motor development are either normal or mildly delayed. Results of routine laboratory tests are normal, as are growth hormone and IGF I levels and chromosomes. Pathogenesis remains unknown. Heterozygotes may show some of the characteristic facial changes.