Down syndrome is the most common chromosomal abnormality. A simultaneous occurrence with Marfan syndrome is extremely rare. We present a case of a 28-year-old female with Down syndrome and a mutation in the fibrillin-1 gene. The patient showed strikingly few manifestations of Marfan syndrome. Although variable expression is known to be present in Marfan syndrome, phenotypic expression of Marfan syndrome in our patient might be masked by the co-occurrence of Down syndrome.