Researchers have paid increasing attention to mental health issues in adults with autism spectrum disorders (ASDs) over the last decades. However, little is known about how rates of clinical referrals, types of mental health diagnoses and treatment in adults with ASDs and intellectual disability have changed. We examined patterns of change in referral trends to specialist mental health services in south London from 1983 to 2000 (N = 137). The majority of the cases (58.4%) did not have a diagnosable psychiatric disorder. Schizophrenia was the most frequent psychiatric diagnosis followed by depression, adjustment reaction and anxiety. There was a significant change in the rate of referrals, an increase in the diagnosable psychiatric disorders over time and a significant reduction of medication at time of referral. There were no significant changes in the use of other therapeutic interventions. The proportion of participants living independently increased. Implications for services and future research are discussed. (Contains 4 tables.)