Depression and Alzheimer's disease are two common and closely intertwined diseases in the elderly. Bio-markers for their early diagnosis would be helpful for clinicians. The brain areas involved in depression, Alzheimer's disease and in olfactory processing overlap, leading to suggestions that olfaction could constitute a potential marker of these diseases. Here, we review the literature in the relevant clinical and olfactory fields, and consider which olfactory measures and factors could serve as markers of these diseases. It has been reported repeatedly that there is an alteration of odor identification in Alzheimer's disease but not in depression. These observations provide strong arguments that this olfactory marker may serve as a complementary tool for the early screening of patients. Odor threshold detection and odor hedonic aspect may constitute complementary markers of the efficacy of depression therapy. However, there are numerous contradictory data and innovative methods are required to investigate whether investigations of olfaction can usefully contribute to routine clinical practice. (C) 2014 Elsevier Ltd. All rights reserved.