There are no specific signs or symptoms in Multiple Chemical Sensitivity (MCS), so diagnosis of MCS depends on a history, QEESI symptom scores and exclusion of other diseases. A gold standard of diagnosis of MCS is a chemical compound exposure test in which changes of symptoms are used to decide the results positive or negative. We have done chemical compound exposure tests to diagnose MCS in 51 patients.Chemical compound exposure tests were done in a special facility (the exposure chamber) in our hospital. Used VOC were formaldehyde or toluene. Maximum exposure concentrations were 0.08 ppm and 0.07 ppm for formaldehyde and toluene, respectively. Forty patients had the tests by a open test manner and 11 patients by a single blinded manner.In the open tests, 18 patients had positive results and 22 patients negative. In 22 patients who had negative results, eleven showed no symptoms by chemical exposures, and other 11 claimed symptoms before VOC went into the chamber. In the single blinded tests, 4 patients had positive results and 7 patients negative.The chemical exposure test is the most reliable test to diagnose MCS, so standardization of the methods of the test is necessary.