There is very limited evidence on the patterns of recreational substance use among adults with Intellectual Disabilities (ID) who have co-morbid mental health problems. In this study we collected clinical and socio-demographic information as well as data on substance use patterns for consecutive new referrals (N = 115) to specialist mental health services for adults with ID in South-East London. The data were recorded from active clinical case notes. About 15% of patients had a history of substance use, however only 8% were currently using substances. Alcohol was the most frequently used substance (80%) followed by cannabis (28%) and cocaine (12%). Overall, substance use was significantly more likely among male patients, those with a mild level of ID and those with a forensic history. Substance use was less likely among patients with autism and more likely among those with schizophrenia spectrum disorders. Logistic regression analyses revealed that those with a forensic history were about five times more likely to have current substance use problems. Male gender was the only predictor for legal substance (alcohol) use. Illicit substance use was about three times more likely among patients with schizophrenia spectrum disorders. The present results highlight the role of illicit substance use as a health risk factor for adults with ID as well as the need to increase awareness within specialist mental health services. (Contains 6 tables.)