It has been suggested that a substantial amount of heat energy could be extracted from dry hot rocks near the earth's surface by circulating a fluid through large hydraulic fractures. However a theoretical analysis of propagation and closure of hydraulic fractures subject to realistic stress gradients indicates an upper limit for the size of a stable fracture. For example, if the vertical gradient of the least principal stress exceeds the unit weight of the fluid by ~100 dynes/cm3, a stable vertical fracture in granite held open by fluid pressure may not be more than ~2¿104 cm high. This limit should be taken into account in the design of dry hot rock heat recovery systems.