Abstract: To improve the homogeneity of the basic magnetic field generated in a nuclear magnetic resonance tomography unit, an iterative shimming method includes the steps of generating a pulse sequence in the form of a bipolar gradient pulse train in the examination volume, and entering the echo signals which are acquired into a measured matrix. A correlation coefficient of the echo signals of at least a part of the rows of the measurement matrix is formed with respect to a reference row. The shim current through at least one shim coil in the tomography apparatus is then varied in a manner causing the correlation coefficient to increase. These steps are repeated until the correlation coefficient reaches a selected value which is large enough to indicate that an adequate basic field homogeneity has been achieved.