Anxiety is a major problem for many people with Asperger's syndrome who may have qualitatively different fears from a non-Asperger's syndrome population. Research has relied on measures developed for non-Asperger's syndrome populations that require reporting past experiences of anxiety, which may confound assessment in people with Asperger's syndrome due to problems with autobiographical memory as are often reported in this group. Experience sampling methodology was used to record real-time everyday experiences in 20 adults with Asperger's syndrome and 20 neurotypical adults. Within-subject analysis was used to explore the phenomenology of thoughts occurring in people with Asperger's syndrome when they were anxious. Comparisons were made with the group that did not have Asperger's syndrome. The Asperger's syndrome group were significantly more anxious than the comparison group. Factors associated with feelings of anxiety in the Asperger's syndrome group were high levels of self-focus, worries about everyday events and periods of rumination lasting over 10 min. People in the Asperger's syndrome group also had a tendency to think in the image form, but this was not associated with feelings of anxiety. The results are discussed with reference to psychological models of Asperger's syndrome, cognitive models of anxiety and implications for psychological therapy for this group.