Background: To describe the characteristics of those with autism spectrum disorder (ASD) treated within a forensic intellectual disability hospital and to compare them with those without ASD. Method: Service evaluation of a cohort of 138 patients treated over a 6-year period. Results: Of the 138, 42 had an ASD. Personality disorders and harmful use or dependence on drugs were significantly lower in the ASD group. The ASD group was less likely to be subject to criminal sections or restriction orders. Self-harm was significantly higher in the ASD group. There were no differences in the length of stay and direction of care pathway. Conclusions: Although the ASD and non-ASD groups differ on clinical and forensic characteristics, their treatment outcomes appear similar. This suggests that the diagnostic category of ASD alone may be inadequate in predicting the treatment outcome. There is a case to identify distinct typologies within the ASD group.