Five individuals in one family, including dizygotic male twins, a half brother and their mother, had Aarskog syndrome (AS). Phenotypic variability is wide not only between mother and sons but also between sibs. Collectively, the affected relatives have the full spectrum of findings seen in AS. Based on analysis of this family and others from the literature, we derive primary and secondary diagnostic criteria for AS. Primary criteria include: short stature, hypertelorism, short nose with anteverted nares, maxillary hypoplasia, a crease below the lower lip, mild interdigital webbing with short and broad hands, short fifth finger with clinodactyly, and shawl scrotum. Secondary criteria include: abnormal auricles with fleshy lobules, posteriorly angulated ears, widow's peak, ptosis, downward slant of palpebral fissures, joint hyperextensibility, broad feet with bulbous toes, cryptorchidism, inguinal hernia, and prominent umbilicus. Literature pertaining to the clinical manifestations and genetics of AS is reviewed and nosology of similar syndromes is discussed.