Depression is a frequent condition in Alzheimer’s disease (AD). The prevalence of depressive symptoms depends on the severity of dementia and the instruments used. Our aim was to assess the prevalence of depression dependent on the severity of dementia by four different scales: The 15-point Geriatric Depression Scale (GDS), the Montgomery and Åsperg Depression Scale (MADRS), the Cornell Scale for Depression in Dementia (CSDD) and the Nurses Observation Scale for Geriatric Patients (NOSGER). The study population consisted of 316 patients with Alzheimer’s disease from a psychiatric out-patients memory-clinic, which was divided into two groups: mild AD (Mini-Mental Status Examination (MMSE) ≥ 18) and moderate to severe AD (MMSE < 18). Additionally, internal consistency and correlation of these scales were calculated. Prevalence of depression ranged between 27.5 and 53.4% in mild AD and between 36.3 and 68.4% in moderate to severe AD. Internal consistency was good in all scales (Cronbach’s alpha .63–.85). For MADRS and CSDD it was independent of the stage of AD, while in GDS and NOSGER internal consistency decreased with severity of dementia. Correlation between the scales was better in mild AD than moderate to severe AD; the best results were obtained for the correlation between CSDD and MADRS in both groups. We conclude that in our study population CSDD and MADRS were the most consistent tools for detecting depression in AD independently of the severity of dementia.