A pair of monzygotic twins with the Marfan syndrome with variable expression is presented. One of the twins, in addition to more severe musculoskeletal and ocular manifestations, had coarctation of the aorta as the cardiovascular manifestation of this symdrome. Analysis of red cell antigens, serum proteins and dermatoglyphic examination suggests a high probability of monozygosity. Accordingly, the variation in expression of this autosomal dominant disorder between the twins is most likely due to the modifying influences of environmental factors. Also noteworthy is the fact that the resected coarctation tissue demonstrated the histopathologic changes characteristic of cystic medial necrosis, and thus served as an additional piece of evidence supporting the diagnosis of the Marfan syndrome. This was of particular importance in view of the absence of any family history of this syndrome and the absence of ectopia lentis or the more typical cardiovascular manifestations in either twin.