A severely mentally retarded 20-year-old male adult with a marfanoid habitus reported since birth and dysmorphic features is described. A younger brother presented the same appearance at birth but died at 2 months of age following a cardiopathy. Despite some clinical differences, his features are very close to the Lujan-Fryns syndrome and to two unrelated girls described by de Die-Smulders et al. (1). This case shows the challenge that clinicians could have while investigating patients with mental retardation and marfanoid habitus for diagnosis and genetic counseling.