As disorders on the autistic spectrum are behaviorally defined, there is no medical test to diagnose autism. The purpose of a medical evaluation is to detect particular etiologies, and manifestations like clinical or subclinical epilepsy or behavior problems that might mandate pharmacologic intervention. Defining a unique syndrome or genetic etiology may benefit other family members, although, currently, specific causes are detectable in only a small minority of individuals on the autistic spectrum. The paper lists elements of the history, examination, and laboratory testing most likely to be informative in clinical practice. Ordering large numbers of tests in the absence of a specific clinical indication is not recommended because it is invasive, wasteful and unlikely to generate useful data. This is not true, of course, in the context of a hypothesis-driven, approved research protocol where collecting standardized data and applying the most up-to-date research technologies is appropriate.