Subjective symptoms are crucial in making the diagnosis of mood disorders and other psychosomatic diseases and in following their courses. To investigate symptoms in natural settings, Stone and Shiffman developed the concept of ecological momentary assessment (EMA) in 1994. EMA is defined as "Monitoring or sampling strategies to assess phenomena at that moment they occur in natural settings, thus maximizing ecological validity while avoiding retrospective recall." Paper-and-pencil diaries are frequently used to perform EMA in medical fields. However, paper-and-pencil diaries are reported to have an intrinsic problem that subjects do not record their symptoms momentarily by completing the diaries after the-fact, which is called "faked compliance." Stone et al. developed an electronic diary (ED) to overcome the disadvantage of a paper-and-pencil diary. In addition, physiological function can also be measured and recorded for a long period in natural settings recently. In this article, we showed an example of EMA data in patients with major depressive disorder and discussed the possibility of "real-time assessment" and "on-site intervention" using an EMA technique.