Although cognitive or intellectual deficits are the most widely researched symptoms of Alzheimer disease (AD) it is often the socio-affective or behavioural, emotional, and personality changes, that the friends and family members of those with AD find most disturbing. The purpose of this research project was to examine the frequency and severity of emotional and behavioural changes in AD and assess whether an individual's premorbid temperament influenced the presentation of these changes. In addition, the study investigated whether individuals with a dual diagnosis of Down syndrome and AD, and those with AD only, differed in terms of the socio-affective and behavioural changes they experienced after the onset of dementia. The primary caregivers of 30 individuals with Down syndrome and AD and 30 individuals with AD only completed the Revised Dimensions of Temperament Survey (DOTS-R; Windle, & Lerner, 1986), the Behavioural Pathology in Alzheimer's Disease Rating Scale (BEHAVE-AD; Reisberg et al., 1987) the Apathy subscale of the CERAD Behaviour Rating Scale for Dementia (Tariot, et al. 1995), and a portion of the Blessed Dementia Scale (Blessed et al., 1968). As well, behavioural observations (n = 26) and personal interviews (n = 6) were undertaken with selected participants. Individuals with Down syndrome were reported by caregivers to experience fewer delusions, and to have lower total scores on the BEHAVE-AD. Behavioural observations of participants revealed that those with Down syndrome were also more physically active than individuals with AD only and engaged in a greater number of both goal-directed (e.g. purposeful activities) and non goal directed (e.g. stereotyped behaviour, socially inappropriate behaviour) behaviours. For both participant groups the behavioural changes of hallucinations, delusions, aggressive behaviour, and anxiety were correlated, and often occurred in the same individuals. This behavioral cluster was found to be associated with the temperamental trait of high activity level, suggesting that individuals with a high premorbid activity level were more likely to experience hallucinations, delusions, aggression, and anxiety after the onset of dementia.