Japanese mistletoe (Viscum album var. coloratum) is an evergreen, hemiparasitic plant that parasitises mainly deciduous trees. Mistletoes are known to have a lower water potential and transpiration rate than host trees and can cause water loss in host trees. In Europe, several studies have investigated the effects of mistletoe infection on host tree physiology or death, but no such studies have been conducted in Japan. In this study, we compared the diameter growth and annual ring carbon isotope composition (δ¹³C) of severely infected, moderately infected and uninfected Fagus crenata in a cool temperate forest. The infection index was classified by counting the percentage of infected branches, and 7 uninfected trees (infection index 0), 14 moderately infected trees (infection index 1) and 10 severely infected trees (infection index 2) were selected as sample trees. In September 2024, 12-mm cores were taken from each tree and the annual ring width for the last 10 years was measured. The annual ring samples were then separated by year, α-cellulose extracted and δ¹³C measured by IRMS. The results showed that δ¹³C of severely infected host trees growing at the bottom of the slope was significantly higher than that of moderately infected or uninfected trees. In addition, severely infected trees had a significantly lower RBAI than moderately infected or uninfected trees. However, there was no significant difference in trees growing higher up the slope. This suggests that severe mistletoe infection may negatively affect the growth of the host trees by depriving them of water and causing a decrease in stomatal conductance on host leaves. This trend has been suggested in many previous studies, but this is the first time it has been shown to occur in a cool-temperate forest. Furthermore, differences in the response to mistletoe infection of trees growing downslope and upslope suggest that the response varies according to topographic conditions, in relation to the distance from streams and water accumulation. This trend suggests that the effect of mistletoe on host trees may vary according to topographic conditions. More research is therefore needed to comprehensively understand the effects of mistletoe infection on host trees, including soil moisture and terrain attributes. We would like to further investigate the possibility that mistletoe may also affect host tree mortality in Japan.