A 4-year retrospective population-based survey was conducted in two cities in Akita Prefecture, Japan, to estimate the incidence rate and the cumulative risk of hospitalization potentially associated with rotavirus diarrhea. At monthly occasions of the 3-year-old checkup, we asked each parent if his or her child had ever been hospitalized because of rotavirus diarrhea. Based on 3-year follow-up of the four consecutive birth cohorts (1996-1999), we calculated the incidence rate and the cumulative risk of rotavirus-associated hospitalizations by the age of 3 years. The incidence rates of rotavirus-associated hospitalization in 1-year-old children in Akita city and Honjo city were 9.7 and 16 hospitalizations per 1,000 children per year, respectively, whereas 1.9% of children in Akita city and 3.3% of children in Honjo city were hospitalized by their third birthday because of rotavirus-associated diarrhea. The burden of rotavirus diarrhea in this region of Japan, and probably across the nation, appears substantially large.