Differences in body size of roe deer (Capreolus capreolus) in four study areas in Denmark were investigated in relation to climate, soils, vegetation types and forest cultures, quality of available forage, population density of roe deer, and other environmental factors. Rumen samples, body weights and measurements, reproductive tissues, and lower jaws were obtained from 73 deer during the study. Body weights and measurements and sex and age data were available from several hundred deer from earlier studies. Field-dressed weight, length of the lower jaw, and femur length showed significant differences among the study areas. The largest deer and those with the most rapid growth were from the two areas that had lower soil fertility, lower ratios of agricultural land to the forest area, lower ratios of perimeter to total forest area, and lower densities of deer. Chemical analyses of washed and unwashed rumen contents indicated that the forage being consumed by the deer was of essentially equal quality on the four study areas. In Denmark, soil quality appears to be a primary factor determining density of roe deer or biomass but does not appear to directly regulate body size. Body size is most directly related to population density. The mechanisms by which density influences body size are apparently social pressures, which affect energy expenditure and food intake, and competition during spring and early summer when foods of high nutritive quality may be limited and physiological demands of the deer are greatest.