A kindred has been studied in which mental retardation and marfanoid clinical features are present in several individuals. The pedigree is consistent with X-linked recessive inheritance. Four affected males aged 12-18 years and four obligate carriers have been identified. Clinical findings in the 4 affected males included a tall slender habitus (3) (the fourth was tall but muscular), a long-narrow face (3), large head (4), highly arched palate (4), small mandible (4), abnormal speech (4), hypernasal voice (3), joint hyperextensibility (3), borderline to large testes (3), pectus excavatum (2), atrial septal defect (1), and a double row of teeth (1). Mental retardation (4) ranged from mild to severe; abnormal behavior included hyperactive and aggressive behavior (2), autistic-like (1) and jovial behavior (1). One and possibly two, males had absence of the corpus callosum. Chromosome studies on all were normal; no marker X was observed. We believe this family probably represents a new form of X-linked mental retardation.