Achondroplasia and trisomy 21 are, within their respective categories, conditions the most frequent genetic diseases found in newborns. The simultaneous presence of both conditions in the same patient, has been however, reported only once in the world literature. In this paper we present a patient affected by both entities (Achondroplasia and Trisomy 21). The clinical findings and the reasons for the rare reported frequency of these cases are discussed.