Reduction of intelligence should be differentiated from interference with the use of intelligence by such non-intellective factors as partial deafness and emotional disturbance. The parents of a retarded child want an assessment, a prediction of the eventual achievement level, and a causal explanation if possible. There are varying degrees of knowledge of causation, from recognition of reduced intelligence only, to an understanding of the mechanism of causation in considerable detail from primary cause to ultimate consequence, as in phenylketonuria or isoimmunization. A diagnosis should be as complete as possible, using available modern techniques of investigation, such as chromatography and cytogenetic studies.AMONG THE RECENTLY DESCRIBED SYNDROMES ASSOCIATED WITH MENTAL RETARDATION ARE: (1) spastic paralysis and congenital ichthyosis; (2) Rud's syndrome; (3) deaf-mutism, infantilism, ataxia and a disturbance of hormone metabolism; and (5) sex-linked deaf-mutism.