BACKGROUND Despite the use of inhaled corticosteroid (ICS) becoming increasingly widespread, many problems related to asthma management still need to be addressed. One of them, obesity, has been reported to exert a harmful influence on asthma control. However, there have been few reports focusing not only on both obesity and its influence on Japanese asthma patients but also on the Japanese definition of obesity, as defined by the Japan Society for the Study of Obesity (JASSO). AIMS & METHODS The aim of this study was to confirm the influence of obesity, as defined by the JASSO, on asthma management in Japanese asthmatic patients. Using data from the Niigata Asthma Treatment Study Group 2008 questionnaire survey, differences between the "normal" group (18.5 kg/m(2) ≤ BMI <25 kg/m(2)) and the "obese" group (25 kg/m(2) ≤ BMI) were analyzed. RESULTS There was a significantly lower step 1 rate (19.4% v.s. 26.8%) and a higher proportion of patients using inhaled salmeterol (43.6% v.s. 35.8%) and leukotriene receptor antagonist (49.8% v.s. 40.8%) in the obese group relative to the normal group, although there were no significant differences in indicators of asthma control, including asthma control test scores. CONCLUSION This study investigated influences of JASSO-defined obesity on asthma severity and management in a clinical setting in Japan. It is possible that there are strong interactions between asthma and obesity, such as obesity causing decreased ICS therapy efficacy and leukotriene (LT)-related inflammation, although further investigation is necessary.