Inclusion Criteria:

1. Male or female participants, three to ten years of age.
2. Meets clinical criteria for an autism spectrum disorder.
3. No antifungal use in the preceding 3 months.
4. Results of pyrosequencing analysis that have identified yeast
5. Each legal guardian must have a level of understanding sufficient to agree to all required tests and examinations. Each legal guardian must understand the nature of the study and must provide written consent to study protocol.

Exclusion Criteria:

1. History of allergic reaction to fluconazole or other azole antifungal agents
2. Current use of terfenadine, cisapride, phenytoin, cyclosporine, rifampin, theophylline, terfenadine, cisapride, astemizole, rifabutin, or tacrolimus
3. History of uncontrolled epilepsy
4. Weight less than 15 kg at screening
5. Presence of a chronic medical condition that might interfere with study participation or where study participation would be contraindicated or clinically significant abnormal baseline laboratory results.
6. Recent (less than two months prior to study entry) initiation of behavior therapy

A child will be excluded at any point during the study if it is revealed that a major change (either addition or deletion) of treatment or service provision has occurred.