In this paper, the effect of anisotropic in situ stress field on the stability of underground openings is investigated. For this purpose, the potential for instability around a tunnel is studied by using elastic and elasto-plastic stress analyses. In the study, unequal principal components of the in situ stress field act in vertical, horizontal and longitudinal directions with respect to the tunnel. For various vertical in situ stress levels, the cases are compared when the longitudinal axis of the opening is oriented parallel or perpendicular to the larger of the horizontal in situ stress components. The analyses have shown that, in an anisotropic in situ stress field, the favorable orientation of an underground opening depends not only on the horizontal in situ stresses but also on the vertical in situ stress.