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During 1968-1971 Japanese encephalitis (JE) surveillance was conducted with WHO-assisted programs in Taiwan area. Emphasis was placed on: (1) active case-finding by hospital visits; (2) blood collection from every patient at the suitable time; and (3) the enforcement of standard diagnostic criteria on the results of hemagglutination-inhibition tests. Each year, approximately 90% of reported patients were etiologically examined. JE virus etiology was established in 277 (1968), 279 (1969), 269 (1970) and 158 (1971) cases. Despite the apparent concentration of cases in several cities, practically all the cultivated plains and basins were established as potentially endemic for JE. JE outbreak is found to have been clearly associated with season, not only in subtropical but also in tropical Taiwan. Each year it occurred consistently in mid-July in the southernmost county, 2-3 weeks after the peak of rice transplantation and its duration was a few weeks. On the western side of the island, although less consistently, JE outbreak seems to occur one to three weeks later than in the south. Outbreak in eastern Taiwan was a mid-September phenomenon for 3 years. Observed seasonal lag, which was significant between two neighboring localities on same latitude, suggests that paddy water management is more directly related to date of outbreak than local climate. Seasonal characteristics further suggests that the area may be separated into eight ecologic subdivisions. JE-virus transmission to a man during April at temperatures below the average of 20 C was discussed.