This paper examines the relationship between monthly Southern Oscillation Index (SOI) and monthly incidences of hemorrhagic fever with renal syndrome (HFRS) and malaria in Anhui Province, China, over the periods 1971-1992 and 1966-1987, respectively. On the basis of monthly data over a 22-year period, results indicated that there were positive and negative relationships, respectively, between the SOI and monthly incidences of malaria and HFRS. The results suggest that the SOI could be used as an index in the study of the association of climate variability with the transmission of such diseases, particularly over larger areas, such as at a provincial or even state level, where averaging rainfall or temperature data across regions is inappropriate.