A review of the 17 previously reported cases of duplication 3p and study of a new patient who has a duplication of the chromosome segment 3p21 leads to pter show a remarkably consistent phenotype among these patients and suggest some generalizations about prognosis. The manifestations include low birth weight, short stature, microcephaly, characteristic "square" face with temporal indentations, hypertelorism and/or telecanthus, epicanthus with a broad nasal bridge and large nasal tip, and down-turned corners of the mouth. Cleft lip/palate and eversion of the lips are common. The jaw is typically small and receding and the neck short. Congenital heart disease, gastrointestinal malformations, abnormalities on intravenous urography, and defective masculinization of the male infants are frequently observed. A predominance of whorls is present on the fingers. Nearly half of the cases died before 6 months. All affected children surviving beyond 1 year have been mentally retarded.