Autism spectrum disorders (ASD) are highly disabling, persistent neurodevelopmental disorders. There are no available treatments for core symptoms of ASD or biologically-based clinical biomarkers. Emerging evidence indicates that levels of brain inflammation are increased in ASD. In particular, recent work implicates hyperactivity of microglial cells, the resident immune cells of the brain. However, the functional consequences of microglial activation remain unknown. This study will measure microglial activation in ASD using positron emission tomography (PET) brain imaging. Adult males with ASD (n=15) and healthy controls (n=15) will be recruited for this study and undergo comprehensive clinical and behavioral baseline assessment. All subjects will then undergo baseline PET imaging using a radiotracer that labels activated microglia. Subjects with ASD will then undergo 12-week open label treatment with minocycline, an FDA-approved antibiotic thought to block microglial activation. PET imaging will be repeated at 12 weeks to confirm target engagement. A subset of control subjects will also undergo repeat PET imaging to determine test-retest reliability. During minocycline treatment, ASD subjects will be evaluated every 2 weeks for safety, clinical impression, behavioral functioning, and measures of cognition. Results will provide important information regarding the relationship between levels of brain inflammation, cognitive and behavioral function in ASD.