Background: Data on the prevalence of developmental disabilities in people who experience imprisonment and on their characteristics are lacking. Methods: The present authors identified adults with developmental disabilities who were released from Ontario provincial prisons in 2010 and a general population comparator group using administrative data. The present authors examined demographic characteristics, morbidity and healthcare use. Results: The prevalence of developmental disabilities was 2.2% in the prison group (N = 52,302) and 0.7% in the general population (N = 10,466,847). The prevalence of psychotic illness, substance-related disorder and self-harm was higher among people in the prison group with developmental disabilities. People with developmental disabilities were more likely to have emergency department visits and hospitalizations in prison and in the year after release. Conclusions: People with developmental disabilities are overrepresented in provincial prisons and have a high burden of disease. Strategies are indicated to prevent incarceration and to improve health.