Functional magnetic resonance imaging typically makes inferences about neural substrates of cognitive phenomena at the group level. We report the use of a single-stimulus BOLD response in the cingulate cortex that differentiates individual children with autism spectrum disorder from matched typically developing control children with sensitivity and specificity of 63.6% and 73.7% respectively. The approach consists of passive viewing of 'self' and 'other' faces from which an individual difference measure is derived from the BOLD response to the first 'self' image only; the method, penalized logistic regression, requires no averaging over stimulus presentations or individuals. These findings show that single-stimulus fMRI responses can be extracted from individual subjects and used profitably as a neural individual difference measure. The result suggests that single-stimulus fMRI can be developed to produce quantitative neural biomarkers for other developmental disorders and may even be useful in the rapid typing of cognition in healthy individuals.