A case-control study to determine the risk factors for human alveolar echinococcosis (AE) was conducted in Hokkaido, Japan. The 134 cases were either patients clinically diagnosed as having AE (N =61) or other individuals found to be seropositive for Echinococcus multilocularis although showing no abnormalities in ultrasound scans (N = 73). Two groups of case-matched controls were employed: one of 670 individuals selected from a list of residents of Hokkaido (group A) and the other of 650 subjects selected from a list of individuals who had been found seronegative in earlier investigations (group B). Because of the long latency of AE, potential risk factors were studied over two different periods: for the 0-10 and 11-20 years before the year of diagnosis/recorded seropositivity. The results of multivariate analyses, in which the cases were compared with each control group, indicated that the rearing of cattle and pigs and the use of well water were significant risk factors and that the use of tap water significantly decreased the risk of an individual having AE. Extension of the piped water supply and health education on improving the disposal of household rubbish (to keep foxes away from houses) should help reduce the risk of AE infection in the study area.