A computerized system (BLIGHTAS) for forecasting the development of rice sheath blight disease was developed based on the ratio of the height of the lesions to the plant height (X, vertical development of the disease) and percentage of the number of diseased hills (A, horizontal development of the disease). In this system, the yield loss of fully ripened kernels in kg per 10 ares (L) is expressed as follows: L=(4I.31X-826.2)A/1000. The X value was calculated from the relation between the daily temperature and relative humidity between hills and susceptibility of leaf sheath. The A value was calculated from the relation between the daily temperature and relative humidity between hills and quantity of sclerotia per unit area of paddy fields. BLIGHTAS designed for use on a personal computer consists of three programs, i.e. analytical program of diurnal variation of temperature and relative humidity, analytical program of temperature and relative humidity, and forecasting program for rice sheath blight disease. It has been confirmed that the yield loss predicted by BLIGHTAS was very close to the actual yield loss caused by the disease. This forecasting system may be applied for the development of new strategies to control the disease. It has been confirmed that the yield loss predicted by BLIGHTAS was very close to the actual yield loss caused by the disease. This forecasting system may be applied for the development of new strategies to control the disease.