This research will investigate whether environmental factors during pregnancy are related to the likelihood that offspring will be diagnosed with autism. Environmental factors such as infections, immune abnormalities, hormones, and smoking will be assessed in pregnancies of 1,000 autism cases and 1,000 healthy children using maternal serum samples that were drawn and stored in nearly all pregnancies in Finland from 1987-2007. Children with autism and healthy children matched to the cases on sex, date of birth, birth place and residence in Finland will be identified from detailed national databases that contain diagnoses on nearly all cases of autism in Finland. Researchers will also test whether there are differences in susceptibility to these environmental factors between boys and girls with autism. This research has the potential to result in a better understanding of the different causes of autism and different developmental pathways that can lead to the illness. The discovery of these causes can lead to prevention of cases of autism by improved prenatal care and to the identification of children who are at risk of developing autism.