In a group of 23 hospital patients aged over 50 with Down's syndrome, psychological testing indicated that significant intellectual deterioration, which was un-related to chronological age, sex, length of hospitalisation, or earlier mental age, had occurred in nine. Clinically, there was no evidence in any patient of active physical illness, focal neurological signs, or dementia, but significant associations were found between intellectual deterioration and decreased visual acuity, hearing loss, and macrocytosis.