Thirty-six de Lange patients aged 5-47 years with a median age of 16 years were tested by the Vineland Social Maturity Scale. Significant physical handicaps affecting the scoring were found in six patients. The social quotients varied from 2.2-99.5. Eighty-one per cent of the patients scored less than 52. Specific retardation was found in verbal communication. Other cognitive skills also showed low scoring, while the patients showed relatively high scores in self-help items. Fifteen patients lived in large institutions for the mentally retarded, three patients lived in hostels or boarding-schools, and one patient lived in her own home. Only two patients, both severely retarded, did not attend any regular daily activity. It is concluded that de Lange patients will not be able to live without support in the community, that they are particularly retarded in verbal communication but that they function relatively well in everyday self-help skills.