Background: Biological measurements that distinguish individuals with autism from typically developing individuals and those with other developmental and neuropsychiatric disorders must demonstrate very high performance to have clinical value as potential imaging biomarkers. We hypothesized that further study of white matter microstructure (WMM) in the superior temporal gyrus (STG) and temporal stem (TS), two brain regions in the temporal lobe containing circuitry central to language, emotion, and social cognition, would identify a useful combination of classification features and further understand autism neuropathology. Methods: WMM measurements from the STG and TS were examined from 30 high‐functioning males satisfying full criteria for idiopathic autism aged 7–28 years and 30 matched controls and a replication sample of 12 males with idiopathic autism and 7 matched controls who participated in …