In order to detect reopening of hydraulically induced axial cracks emanating from borehole, we developed a new dilatometer tool and proved it in the laboratory. The tool consists essentially of a packer around which deformation transducers sensitive to tangential strain are affixed. The packer is inflated to touch the borehole wall, and then pressure is increased step-wise and the resultant strains recorded. Initially, the circumferential strain developed around the borehole is radially symmetric. However, this becomes strongly asymmetric orice the cracks begin to open. Thus, the records of strain at different locations around the borehole indicate clearly the packer pressure at which the cracks begin to open. If the borehole is vertical, the detected pressure at crack opening allows us to estimate the maximum horizontal stress on condition that the minimum horizontal stress is estimated from the crack closure pressure observed in hydrofracturing test.