A horizontal crack, in the crest gallery of the arch-gravity Jablanica dam, was observed 40 years ago. Since then, the crack has propagated across a substantial part of the gallery towards the downstream face with a continuous and slow increase in width. The problem has been investigated with the help of a DIANA 3D finite-element dam-foundation model. Calibration of the thermal and elastic parameters of the model is conducted after a sensitivity analysis with an almost perfect fit of calculated to measured pendulum movements. The results of time dependent thermo-stress analyses showed that the concrete temperature change, at a certain period of the season, causes principal tensile stresses almost perpendicular to the plane of the observed crack. These stresses are sufficient to cause an opening of poorly executed construction joints, considering a year by year redistribution of tensile forces to uncracked parts of the tensile surface as the crack propagates. However it has been proved that the dam is safe in all dynamic and static summer and winter load combinations. This situation is not due to the tensile capacity of the uncracked part of tensile surface but because crack faces are in compression at the time of all critical load combinations.