Objectives To evaluate the effect of Care Quality Commission external inspections of acute trusts on adverse event rates in the English National Health Service. Methods Interrupted time-series analysis including all acute NHS trusts in England ( n = 155) using two control groups (new versus historical inspection regime and trusts not inspected). Multilevel random-coefficient modelling of (1) rates of falls with harm and (2) pressure ulcers, from April 2012 to June 2016, was undertaken using the new, resource-intensive regime of Care Quality Commission inspections as an intervention. Data used in the model included dates and type of inspection, patient safety indicators, demographic characteristics and financial risk of hospitals. Results In one year, Care Quality Commission inspected 66 acute trusts (42% of all English trusts) using their new regime and 46 (30%) using their previous one. Prior to inspections being announced, rates of falls with harm and pressure ulcers were improving in both intervention and control hospitals. The announcement of an inspection did not affect either indicator. After inspections, rates of falls with harm improved more slowly, and pressure ulcer rates no longer improved for trusts inspected using both regimes. Conclusions Neither form of external inspection was associated with positive, clinically significant effects on adverse event rates. Any improvement happening before the announced Care Quality Commission inspections slowed after the inspection.