A 3-year-old girl, her mother, and maternal uncle had microcephaly and mental retardation. Their facial appearance is characterized by deep-set eyes, short philtrums, and a "beaked" nose. The mother and uncle live in an adult foster care facility because of mental retardation. The 3-year-old girl has a developmental quotient of 55. Mother has normal phenylalanine level and the child's chromosomes are normal. This appears to be a first report of a autosomal dominant form of microcephaly associated with mild to moderate mental retardation in contrast to absent or mild mental retardation described in earlier reports.