We describe a 34-year-old woman with mental retardation, short stature, macrocephaly, a "coarse" face, hoarse voice, and redundant skin with deep palmar and plantar creases who had evident Costello syndrome. Lacking papillomata, she had wart-like lesions of the skin. The previously reported patients with Costello syndrome are reviewed. Costello syndrome is probably an autosomal dominant disorder, either caused by a mutation in a single gene or by microdeletion.