This study was directed to the Peizootics of Newcastle disease in Korea. Since 1960 New-castle disease epizootics have been observed at an interval of one or two years. The frequency of Newcastle disease outbreaks was related to the amount of the vaccine used, being the more sufficient vaccine produced the fewer outbreaks. Regional distribution of Newcastle disease outbreaks was most frequent in the capital area, being the center of poultry raising and Poultry product marketing, and infrequent in areas distant from the capital. Spreading of Newcastle disease appears to be mainly due to the movement of infected chickens. More than 60 percent of the total outbreaks of Newcastle disease diagnosed at the Veterinary Research Institute in Anyang were observed in young chickens aged less than 60 days. Improper vaccination, interference by maternally derived antibody with the immune response following vaccination, ana immaturity of immune systems of young chickens might have contributed to the results. This view was further supported by the findings that 23 percent of the total flocks experienced Newcastle disease were not vaccinated at all and, in vaccinated flocks, virtually all of the flocks were vaccinated by the drinking water method, which has been known less effective compared to individual application or spraying methods.