Cedar pollinosis in Japan affects nearly 25 % of Japanese citizens. To develop a treatment for cedar pollinosis, it is necessary to understand the relationship between the time of its occurrence and the amount of airborne cedar pollen. In the spring of 2009, we conducted daily Internet-based epidemiologic surveys, which included 1453 individuals. We examined the relationship between initial date of onset of pollinosis symptoms and daily amount of airborne cedar pollen to which subjects were exposed. Approximately 35.2 % of the subjects experienced the onset of pollinosis during a one-week interval in which the middle day coincided with the peak pollen count. The odds ratio for this one-week time interval was 4.03 (95 % confidence interval: 3.34-4.86). The predicted date of the cedar pollen peak can be used to determine the appropriate date for initiation of self-medication with anti-allergy drugs and thus avoid development of sustained and severe pollinosis.