The invention relates to a method and a unit for automatically adjusting a collimator (6). In this connection, a region (9) of interest inside the body is determined in an application-specific way from an analysis of first X-ray pictures, and the collimator (6) is then adjusted thereon. The region (9) of interest can, in particular, be chosen to be large enough for the irradiation field to cover all those positions of an organ (10) of interest that occur as a result of heartbeat and/or respiration. Preferably, a movement estimate' is undertaken during a current examination in order to be able to readjust the collimator (6) if necessary. If the region of interest cannot be localized, the collimator (6) is opened to a standard adjustment.