Summary

A male neonate with maternally inherited Marfan syndrome was also diagnosed with Down syndrome at 3 weeks of age. To our knowledge this is the first described case in the literature of the co-occurrence of Down syndrome and Marfan syndrome in a neonate. The diagnosis of Down syndrome was delayed and we hypothesise that Marfan syndrome had masked the usual phenotypic features of Down syndrome. The phenotype of this child is intriguing and has lead to speculation of the possible interaction of the two syndromes.