Astrometric gravitational microlensing is an excellent tool to measure the mass of a star. Even though astrometric microlensing events with measurable deflections are extremely rare, there is one big advantage compared to photometric microlensing events: Astrometric microlensing events can be predicted, based on highly accurate positions, proper motions and parallaxes of lens star and source star. Through a time series of very precise measurements of the lensed background star’s position during the passage of the foreground lensing star it is possible to determine the mass of the lensing star with an accuracy of a few percent. In this talk I will present the analysis of the recent published Gaia data release 2 (DR2) and the prediction of a number of astrometric microlensing events in the near future. In particular I will present an ongoing microlensing event caused by the high proper motion star Luyten 143-23.