Alzheimer's disease (AD) is the most common progressive neurodegenerative disease. Today, AD affects millions of people worldwide and the number of AD cases will increase with increased life expectancy. The AD brain is marked by severe neurodegeneration like the loss of synapses and neurons, atrophy and depletion of neurotransmitter systems in the hippocampus and cerebral cortex. Recent findings suggest that these pathological changes are causally induced by mitochondrial dysfunction and increased oxidative stress. These changes are not only observed in the brain of AD patients but also in the periphery. In this review, we discuss the potential role of elevated apoptosis, increased oxidative stress and especially mitochondrial dysfunction as peripheral markers for the detection of AD in blood cells especially in lymphocytes. We discuss recent not otherwise published findings on the level of complex activities of the respiratory chain comprising mitochondrial respiration and the mitochondrial membrane potential (MMP). We obtained decreased basal MMP levels in lymphocytes from AD patients as well as enhanced sensitivity to different complex inhibitors of the respiratory chain. These changes are in line with mitochondrial defects obtained in AD cell and animal models, and in post-mortem AD tissue. Importantly, these mitochondrial alterations where not only found in AD patients but also in patients with mild cognitive impairment (MCI). These new findings point to a relevance of mitochondrial function as an early peripheral marker for the detection of AD and MCI.