Clinical cases of Murray Valley Encephalitis in southeast Australia have tended to occur in summer and autumn following extended periods of above average rainfall over most of eastern and northern Australia. The Southern Oscillation, an important mode of climatic fluctuations over the Indian and Pacific Oceans, is closely related to eastern and northern Australian rainfall. The Southern Oscillation has been used previously to develop methods for predicting rainfall fluctuations over Australia and their biological and economic impacts. The relationship, therefore, between the Southern Oscillation and Murray Valley Encephalitis in southeast Australia was examined. Darwin atmospheric pressure, an index of the Southern Oscillation, was found to be well below average during the autumn, winter and spring preceding the occurrence of Murray Valley Encephalitis. It is suggested that this relationship can be used to provide a simple, objective early warning system.