- We studied the nature and causes of delusions after infarction of the right cerebral hemisphere. Delusions involved orientation in time and place, events in the recent past, and the identities of familiar individuals. Some were transient and disorganized, and others were more fixed. No particular site in the right hemisphere was associated with particular delusions. The different delusions may be varied manifestations of a basic disorder of thought and memory. The major determinant of the delusions was cerebral atrophy. Age, location, and size of lesion had no major effect. The superimposition of a new right hemisphere lesion on a diffusely atrophied brain seems to cause delusions that can be expressed verbally because the left hemisphere is relatively preserved.