ABSTRACT.There are four administrative areas (Health and Social Services Boards) in Northern Ireland. Two prevalence surveys, carried out several years apart, show that there is a significant geographical gradient in the rates of severe mental handicap (IQ<50). The rates in the 20‐29‐year‐old groups in the Eastern and Northern Boards range between 4.07 and 4.82 per thousand; comparable rates in the Southern and Western Boards range between 5.17 and 6.37. A similar gradient was discernible in the distribution of Down's syn