This cross-sectional study examines relationships among the constellation of psychiatric syndromes in Alzheimer's disease (AD) as a function of dementia severity in 1155 patients with probable AD. The frequency of major depression decreased in severe stages, while agitation, aggression, and psychosis were more frequent in late stages. Major depression was associated with anhedonia, sleep disorders, depressed mood, low self-esteem, anxiety, and hopelessness in mild/moderate and severe stages. Agitation was associated with aggression and psychosis in mild/moderate stages, and psychosis was associated with aggression in moderate/severe stages. In addition, there was a constellation of psychiatric symptoms (e.g., anxiety, wandering, irritability, inappropriate behavior, uncooperativeness, emotional lability) associated with agitation, aggression, and psychosis, which varied according to the severity of the dementia, suggesting a progressive deterioration of frontal-temporal limbic structures. Education and race were independently associated with psychosis. However, while education was associated with psychosis in mild/moderate stages, race was associated with psychosis in moderate/severe stages.