It is often difficult to decide whether a preschool child who does not communicate effectively has a developmental disorder limited to language, has behaviors 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, and chromosome and chemical tests rarely provide an answer. Careful attention to the child's language and behavior, 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 to have considered the problem of making a diagnosis among these often confusing and not easily assessable young children. It considers explicitly the practical consequences of using different criteria for making a diagnosis.