There have been many theoretical studies to predict the stress fields around the tip of a growing fatigue crack. However, until recently the highly-localized, small scale nature of the stresses has meant that direct measurement has not been possible. With the current generation of synchrotron X-ray sources, sub-millimetre sampling dimensions are now possible, and it has become possible to evaluate directly the stresses at the tip of a fatigue crack and to see how the stresses evolve as the result of an overload, for example. In this paper we present results of synchrotron X-ray diffraction analysis of the stress fields around a fatigue crack in aluminium alloy 5091 (Al-Mg-Li-C-O); this is a dispersion-strengthened alloy with a fine grain size, which makes it ideal for such experiments. Compact tension (CT) specimens were prepared with constant amplitude fatigue loading. The energy dispersive X-ray diffraction (EDXRD) technique was used for measuring strains around the crack tip along the mid thickness of the specimen under in-situ loading. The measurement was carried out at the ESRF (European Synchrotron Radiation Facility), Grenoble, France on the ID15A beam line. The experimental crack tip stresses have been compared with the analytical fracture mechanics solution.