Over-representation of people with learning disability in prisons has been demonstrated in many Western jurisdictions. This was the first comprehensive research in a UK prison. The research used a random 10% sample of a prison population (n = 140). A semi-structured interview, the Wechsler Adult Intelligence Scale-III (UK version) and the Vineland Adaptive Behavior Scales (VABS) Interview Edition were administered. On the Wechsler Adult Intelligence Scale-III, UK version (WAIS-III (UK)), 7.1% of participants achieved standard scores below 70 and a further 23.6% were in the 70-79 (borderline) range. The VABS results indicated that 10.1% had standard scores below 70 and a further 33.3% were in the borderline range. The results indicate that the prevalence of intellectual (or learning) disability as measured on either or both of the WAIS-III (UK) or the Vineland Adaptive Behavior Scales is higher than previous research in the UK has indicated. The results have implications for provision of services in prison and post-release.