FMS affects 1.5 – 2.0 % of the population in industrialized countries around the world. Although FMS is characterized with indications such as chronic and widespread pain, fatigue, sleeping disturbance, and physical and psychological impairment, there is no concrete structural and/or pathological evidence relevant to the disease. In this respect, many physicians do not accept FMS as a disease, although patients suffer real physical and psychological dysfunction. The diagnosis of FMS is classically based on an examination of the pain-sensitive points in the body, which is rather subjective in nature. Furthermore, FMS often manifests comorbidity with RA or SpA, both of which exhibit a similar sign—a great pain. This facet has not only made a sound diagnosis for FMS difficult, but also confounded FMS with RA and/or SpA. Therefore, more rapid and precise diagnostic markers for FMS that simultaneously distinguish it from RA and SpA have long been sought. This study has opened a novel avenue for the objective diagnosis of FMS, which has up until now been difficult due to the absence of concrete structural and/or pathological parameters, the ignorance of physicians, and the problem in differentiating FMS from RA and/or SpA. Rapid diagnosis of FMS will ameliorate quality of life for the patient. It is also important to keep in mind that most research on novel drug development, including basic research, particularly in immunology, is performed on mice, for which MAIT cells are quite rare, and not on humans for which they are abundant. Exploiting the function of MAIT cells in health and disease will shed light on the etiology and/or pathology of FMS, which in turn will engender the advent of revolutionary drugs.