Special interests are frequently developed by individuals with autism spectrum disorder, expressed as an intense focus on specific topics. Neurotypical individuals also develop special interests, often in the form of hobbies. Although past research has focused on special interests held by children with autism spectrum disorder, little is known about their role in adulthood. The current study investigated differences in the content, number, and specificity of the special interests held by adult individuals with autism spectrum disorder and neurotypical individuals, using Internet discussion forums as a data source. Quantitative analysis of forum posts revealed significant differences between the diagnostic groups. Individuals with autism spectrum disorder reported having more interests in systemizing domains, more specific interests, and a greater number of interests overall than neurotypical individuals. Understanding special interests can lead to the development of educational and therapeutic programs that facilitate the acquirement of other important social and communication skills.