It is often difficult to decide whether a preschool child who does not communicate effectively has a developmental disorder limited to language, has behaviours that indicate autism with or without mental deficiency, or across-the-board mental deficiency without autistic features. All of these disorders denote abnormal brain development, but standard medical and neurologic examinations and tests such as brain imaging, EEGs, chromosome and chemical tests rarely provide an answer. Careful attention to the child's language and behaviour, ability to play creatively, and analysis of neuropsychologic tests and language are much more informative for arriving at a correct diagnosis. This is one of the largest multidisciplinary studies which has considered the problem of making a diagnosis among these often confusing and difficult to test young children and considers explicitly the practical consequences of using different criteria for making a diagnosis.