Mycophenolate mofetil (MMF) in combination with cyclosporine (CsA) or Tacrolimus (TAC) has been show to be a potent immunosuppressive agent. The authors assessed the mycophenolic acid (MPA) plasma levels achieved in clinical practice and evaluated the effect of concomitant administration of CsA and TAC . One hundred forty transplant patients (kidney: 120 and lung: 20) received a triple immunosuppression regimen of CsA or TAC, prednisone and MMF. Twenty-two renal transplant patients received double therapy with MMF and prednisone. There was no correlation between MMF dose and MPA trough concentrations (r = -0.0657). The medians (range) of the MPA dose-to-concentration ratio (D/C) in the CsA and TAC groups were 0.90 (0.11-8.33) and 0.56 (0.11-14.3), respectively (p < 0.0001). According to the post transplant period (1-3, 4-6 and >6 months), D/C values were significantly lower in patients receiving MMF and TAC than those receiving MMF and CsA in all three periods. MPA levels in patients treated with MMF and CsA were significantly lower than those obtained in double therapy. The D/C ratio in CsA-treated patients, increased significantly (p = 0.0005) when CsA level increased. There was no relationship between D/C ratio and TAC blood concentrations. These results suggest that CsA exerts an influence on MPA trough levels, although further work is required to characterize the mechanism of interaction.