Background: Autism spectrum disorder (ASD) refers to a group of conditions characterized by problems with socialization (sharing emotions and understanding how others feel), communication (both spoken and unspoken, such as pointing, eye contact, and smiling), and behavior (such as repetitive or restrictive activities). The prevalence of ASD is increasing worldwide, in part due to the increased recognition and early diagnosis of these conditions. Some children with ASD are severely affected, while others have very mild symptoms. Furthermore, children with ASD have a high incidence of associated problems such as restricted diet, seizures, sleep and behavior problems, constipation and abdominal pain, and chromosomal abnormalities. Although numerous studies have attempted to identify causes of ASD, there is no known etiology for the majority of cases. The identification of reliable early predictors for the development of ASD might allow earlier diagnosis and treatment and can improve outcomes. In addition, better understanding of conditions associated with ASD would allow families and their doctors to more easily understand what to expect in children with ASD and subsequently improve the medical care and quality of life for these patients. Objective/Hypothesis: The purpose for this study is to more fully understand the causes and associated medical conditions or problems that occur in children with ASD. We believe that there are specific causes of ASD yet to be identified and that various problems associated with ASD are significant but poorly understood. We seek to identify causes of autism and better define associated conditions to improve care for patients with ASD. Specific Aims: We will look at various conditions existing during pregnancy, delivery, and after birth, and whether any of these conditions may contribute to the development ASD. We will look specifically at infections during pregnancy, factors such as season of conception or birth, medication use during pregnancy, problems with the delivery or after birth, and jaundice in newborn babies. We will also better quantify the medical, behavioral, and genetic conditions that children with ASD have, which may be helpful in determining the best way to treat them. Study Design: We will look at a large number of patients within the Military Health System (MHS), which includes data on outpatient clinic visits, prescriptions, inpatient admissions, and laboratory data of all military members and dependents treated in both military and civilian medical facilities. We will look at children with ASD between 2 and 8 years old and compare them to a group of children without ASD. This will allow the prevalence of any identified risk factors and medical conditions to be determined. Impact: Approximately 9,000 children with ASD will be studied, which would be the largest of any US study and unique in its ability to link child and parent medical data. We hope to identify and confirm true risk factors for ASD, which could lead to ASD prevention or earlier identification. We also hope that increasing understanding of the various associated medical conditions or behavior problems in children with autism will improve their medical care. Innovation: The large, all-inclusive, database of children with universal healthcare offers a unique opportunity to look for differences between children with and without ASD. This will allow the identification of rare risk factors and associated conditions. This study will be run by a group of specialized academic pediatric doctors and health services research specialists with first-hand experience with research, as well as the identification and management of children with ASD.