Cyclosporine (CsA) and some of its metabolites (M9, M17, M18, M21) have been determined by means of an LC-MASS method in eight psoriatic patients developing nephrotoxicity. In comparison with a control group (15 psoriatics who after the same period of time, with the same daily dose, did not develop nephrotoxicity) they showed an increase of CsA metabolites, especially M17. Because M17 blood concentrations in the nephrotoxic group tended to be higher than in the control group from the first week of treatment we suggest that M17 might be considered a marker of ongoing nephrotoxicity.