This study investigated whether children with autism spectrum disorders (ASDs) exhibit formal thought disorder (FTD), and whether this is related to ASD symptoms, executive control, and anxiety. Participants aged 8–17 with ASDs exhibited significantly more illogical thinking and loose associations than matched typically developing control subjects. In participants with ASDs, illogical thinking was related to aspects of cognitive functioning and to executive control. Loose associations were related to autism communication symptoms and to parent reports of stress and anxiety. When FTD is present in ASDs, it generally is not a co-morbid schizophrenia symptom, but is related to pragmatic language abnormalities found in ASDs. The clinical and neurobiological significance of this work is discussed.