People with developmental disability have higher health care needs and lower life expectancy compared with the general population. Poor quality of care resulting from interpersonal and systemic discrimination may further entrench existing inequalities. We examined the prevalence of five avoidable in-hospital patient safety incidents (adverse drug reactions, hospital-acquired infections, pressure ulcers, postoperative pulmonary embolism or deep vein thrombosis, and postoperative sepsis) for four developmental disability groups (people with intellectual disability, chromosomal abnormalities, pervasive developmental disorders, and congenital malformation syndrome) in the English National Health Service during the period April 2017-March 2019. We found that the likelihood of experiencing harm in disability groups was up to 2.7-fold higher than in patients without developmental disability. Patient safety incidents led to an excess length-of-stay in hospital of 3.6-15.4 days and an increased mortality risk of 1.4-15.0 percent. We show persisting quality differences in patients with developmental disability, requiring an explicit national policy focus on the needs of such patients to reduce inequalities, reach parity of care, and lower the burden on health system resources.