In a sample of 178 subjects with Alzheimer's disease, diagnosed by clinical criteria (NINCDS/ADRDA), delusions had occurred in 16% of the sample since the onset of the illness and been present within the last 12 months in 11%. Simple delusions of theft and suspicion were the most common types and a greater proportion of men suffered delusions of theft. Subjects with other types of delusion had relatively well preserved lateral ventricular size and basal ganglia calcification. Twenty per cent of the group had experienced persecutory ideation short of delusions since the onset of the illness. Cognitive function at entry to the study and cognitive deterioration over the succeeding 12 months was not influenced by the presence of disorders of thought content.