Although the links between autism and certain diag nosable conditions are often convincing, the number of individuals who are within the autistic spectrum and have known genetic or non genetic conditions is only a small percentage of the whole, and an association with autism is not universal in any one of the diagnosable medical or genetic conditions mentioned. In population-based studies of children with autism, such conditions account for a small minority, probably <10%. However family studies indicate that genetics play the major causative role in most individuals with “idiopathic” autism. Since all patients with a chromosomal imbalance are dysmorphic, the association of ASD with a facial dysmorphism seems to be a good indication for chromosomal anomaly screening. Find ings such as micro- or macrocephaly, abnormal finger digit ratios, and other dysmorphic features, are associated with various developmental abnormalities of the brain and mental retardation. Because the frequency of specific diagnoses is highest in children with cognitive impairment or congenital anomalies, it is recommended that for routine clinical care, extensive testing be limited to those with a suspicious family or medical history, mental retardation, or dysmorphology and to families who wish to have additional children, as different genetic disorders have different recurrence risks. Although prenatal diagnosis is possible for de- fined disorders such as FXS, there is no prenatal test to identify “idiopathic” autism. Given the recurrence rate of 2 to 8% in siblings of affected children, and that the initial diagnosis of autism is made between 1 and 4 years of age, it is especially important to offer parents information about their recurrence risks before they conceive another child.