There is an insufficient number of field studies on the impact of wind farms on ungulates despite the rapid expansion of world wind energy. The results of previous studies suggests a possible impact of power plants, but they are divergent and do not take into account the size of wind farms. We examined the impact of wind farms on the stress levels in roe deer based on seven wind farms of various sizes (of 12–27 turbines) in eastern Poland. Fecal cortisol concentration was assessed with the ELISA method in droppings collected during the winter period. We found that the roe deer exhibited an elevated stress level in the area of larger farms, but such response was not found in the case of smaller wind farms. The roe deer were also characterized by a higher stress level in areas of wolf predation, but this factor was less important. Both the area of the farm and the number of turbines explained the phenomenon of the increased stress in the roe deer. We estimated 824 ha or 18 turbines as a threshold level of the impact of wind farms on the cortisol concentration in the roe deer. We conclude, that turbines should be concentrated in the smallest possible area. In such conditions, roe deer will probably be able to find appropriate refuges. We recommend further studies on wind farms older than 4 years and the distribution of turbines.