It is recognized that there may be numerous medical hazards associated with the maintenance of relative humidity below 50 percent. These include the danger of producing hypothermia in patients, especially during long operative procedures; the fact that floating particulate matter increased in conditions of low relative humidity, and the fact that the incidence of wound infections is minimized following procedures performed in those operating rooms in which the relative humidity is maintained at the level of 50 to 55 percent. So, we have checked room temperature and relative humidity of operating theater in Hanyang University Hospital during 1 year from January, 1991. The results were as following; 1) Room temperature wae maintained at the level of 23.6±0.8 to 27.7±1.3 degrees C, and it was somewhat lower in shadowy rooms than in sunny ones. 2) Relative humidity was highest level in July, and it was maintaiaed at the level of 41.7±4.9 to 47.0±7.7%. Generally, it was decreased unsignificantly in order of ICU, recovery room, newbuilt shadowy rooms, old shadowy rooms, and sunny rooms. 3) Relative humidity was significantly decreased to 2.4±3.8-28.9±5.0% in spring, autumn, and winter. 4) Absolute humidity was maintained at the level of 11.6±1.7 mgH2O/L in summer, and l.l±1.4 mgH2O/L in winter. With above results, we recognized that room temperature of operating theater was maintained at the level of recommending degree, but relative humidity(percent) was too low to be suitable for operation and/or special patient care. This means that ventilation of operating theatre should be installed with proper temperature and relative humidity controlled units.