Background It is unclear whether the substantial decline in number and duration of admissions for patients with intellectual disability (ID) have occurred uniformly over time with respect to age, gender, severity of disability, legal status and location of treatment. Method A retrospective analysis of NHS (National Health Service) admissions for ID and use of NHS ID beds in England between 1998/9 and 2007/8. Results NHS admissions for ID halved from 37 736 to 18 091, and admissions with a primary diagnosis of ID to beds reduced by 71% from 21 866 to 6420. This reduction was most marked among children with the result that the average age of those admitted increased from 26 years to 36 years. Mean length of stay shortened except for mild ID where it increased from 131 days to 244 days (the median increased from 6 days to 32 days). There was an 18% reduction in the number of patients with ID who were legally detained to NHS facilities but a 170% increase in those to private facilities (from 202 to 545). Conclusions The number of patients with ID admitted to NHS facilities, especially children, has fallen dramatically. There has been a marked shift towards legal detentions to private facilities. The most notable finding was the increased duration of admissions for those with mild ID, possibly indicating that substituting mainstream for specialist services for this group has had negative consequences.