Precise assessment of stress is an imminent issue to deal with stress-related social, medical and psychological problems. Psychological stress is known to stimulate the neuroendocrine, sympathetic nervous, and immune systems. By analyzing mRNA expression levels in leukocytes, which express receptors for hormones, neurotransmitters, growth factors, cytokines, and other stress related signals, levels of stress may be adequately measured. In a series of studies, our group has developed a cDNA microarray specifically designed to measure the mRNA levels of stress-related genes in peripheral blood leukocytes. This microarray enabled us to sensitively detect the response to psychological stress. In addition, our preliminary study suggests that the array could differentiate patients with depression from sex- and age-matched control subjects.