Two unrelated children with features of the Cornelia de Lange syndrome, including mental retardation, growth retardation, glaucoma, and a similar facies, had duplications corresponding to bands q25 leads to q29 of chromosome 3. These patients were compared to others with duplications of chromosome 3 and to a large series of patients with Cornelia de Lange syndrome. Diseases that sometimes involve characteristic chromosomal changes but have normal chromosomes in other instances are discussed.