Abstract :
Autism is a developmental disorder in children that is complex and severe, usually seen before the age of 3 years, unable to communicate and express feelings and desires. As a result of behavior and relationships with other people become disturbed, so that this situation will greatly affect the child's further development. Thus, it is often found that the characteristics overlap with several other developmental disorders. The gradation of the manifestation of the disorder is also very wide, ranging from severe to mild. On the one hand there are individuals who have few symptoms. Then an application of an expert system for therapy for autistic children was developed at the Riau Province Autism Service Center (PLA) using case-based reasoning in which to determine the results of therapy to be carried out by autistic children using the case-based reasoning method and then using weighted nearest neighbor similarity. As well as user testing and correspondent answers have good performance with a value of 92.5% so that this application can be applied.