Depression (D) in people with Alzheimer's disease (AD) is common. Coexistence of D and ?D has recently become one of the most interesting problems for gerontopsychiatrists due to high frequency of this co-occurrence in clinical picture of ?D and due to difficulties in differentiation between D and dementia in old age. It's important to study an influence of depressive disorders on progression of dementia. Two groups of AD patients with and without symptoms of D were compared. The comparative analysis shows that presence of D correlates with earlier ?D onset, which thought to be a probable trigger factor for AD manifestation or the earliest symptom of developing neurodegeneration. The data showed that D in dementia increased the speed of progression of cognitive decline. Furthermore, presence of D leads to earlier institutionalizing of the patients and increases the mortality rate of patients with senile and presenile types of ?D at the mild and moderate stages of dementia. Selective inhibitor of serotonin reuptake may be considered to treat D with AD. The side effects of tricyclic antidepressants can be harmful for people with AD.