For the first time, the KLL Auger spectrum of La (Z = 57) following the electron capture decay of radioactive 139,141Ce in a solid source was measured. The all nine basic spectrum components were resolved and their energies and relative intensities were determined and compared with theoretical expectations. The absolute energy of 27 383.8 ± 2.2 eV derived for the dominant KL2L3(1D2) transition was found to be higher by 7.7 ± 2.2 eV (i.e. more than 3σ) than a value predicted by the widely used relativistic semi-empirical calculations by Larkins. The possible reasons for this discrepancy are discussed. Despite of less accuracy of some measured data, the predicted strong influence of the relativistic effects on the KL1L2(3P0) transition intensity was nevertheless proved.