In order to add to the knowledge on adult phenotypes of metabolic disorders associated with intellectual disability (ID) and to evaluate criteria for recommending metabolic testing of adolescents and adults with unexplained ID, the authors analyzed retrospectively the outcome of metabolic investigations performed during a 10-year period on 256 people with ID aged 16 years or older from 245 families. A metabolic disorder was found in 13 people from 10 families, in some cases only after a second investigation. One diagnosis (homocystinuria) would have been missed if clinical selection had been performed. The present cases illustrate a broad clinical spectrum. The authors propose an adapted list of clinical findings that may indicate the need for metabolic testing of adolescents and adults with ID. The study contributes to the knowledge of the adult phenotype of metabolic disorders. A metabolic diagnosis as the cause of ID may lead to the adaptation of care procedures, often defines a low recurrence risk for the progeny of siblings and means relief from uncertainty for family members.