Hair samples have been increasingly used for measuring the long-term stress mediator cortisol. However, since hair is not always available, nails as a keratinized matrix seem to be a possible alternative to hair. In order to measure cortisol and cortisone in nails, an LC-MS/MS method was developed and validated by using 13C3-labeled analytes as surrogate analytes. Both analytes were measured in ESI negative mode as formic acid adducts. Different sample preparation techniques were tested and single step extraction in methanol was established for determination of cortisone and cortisol in nails. The method was successfully validated. Limits of detection (LOD) and limits of quantification (LOQ) of 0.5 pg/mg and 1.0 pg/mg for cortisol and cortisone were achieved. The calibration curve was linear up to a concentration of 500 pg/mg. Recovery was good for both analytes and showed values of over 50%. Matrix effects with ion suppression occurred for both substances but could be corrected by the use of internal standard. Accuracy and precision were in the accepted range of $\pm$ 20% for both substances. The method was successfully applied to determine cortisol and cortisone concentrations in authentic nail samples. Cortisol and cortisone concentrations varied significantly among different fingernails, being highest in the little finger nails and lowest in the thumbnails. No correlation could be found between hair and nail cortisol as well as between hair and nail cortisone concentrations. Furthermore cortisol and cortisone concentrations were significantly higher in hair.