The purposes of this study were to clarify the characteristics of physician-diagnosed MCS patients in Japan. Thus, based on the medical records of 106 patients diagnosed with MCS according to both the 1999 Consensus and the Japanese diagnostic criteria for MCS, we analyzed demographic characteristics (sex and age), onset factors, co-morbid allergic diseases. In addition, subjective symptoms were evaluated using the QEESI®, and we compared the QEESI® scores with those of four MCS patient groups in the U.S. reported by Miller and Prihoda (1999b). Female patients accounted for 74.0%. The peak age in males was 30-34 years, whereas females showed a wide age distribution with three peaks, 30-34, 45-49 and 60-64 years. The most common MCS onset factors included indoor air contaminants by new construction or renovation of home or work place (63.2%), followed by exposure to chemicals in workspace or hobby (26 3%). In addition, co-morbid allergic disease was presented for 84.0%.