This study was designed to compare the enzyme multiplied immunoassay technique (EMIT) and a specific ;liquid chromatography-tandem mass spectrometry (LC-MS/MS) method for the determination of mycophenolic acid (MPA) concentrations in plasma samples collected from renal transplant patients receiving mycophenolate mofetil in association with sirolimus and corticosteroids. Ten-point blood concentration-time profiles, between pre-dose (C0) and 9-hour post-dose, were obtained on day 7, day 14, months 1, 2, and 3 posttransplantation in 8 patients. MPA plasma concentrations were measured simultaneously with both EMIT and LC-MS/MS. Higher concentrations were obtained with EMIT. The mean (+/-SD) absolute difference was +0.84 +/- 1.93 mg/L, which corresponds to a significant overestimation of 18.7 +/- 2 6.8% with EMIT, with variations depending on the time elapsed since transplantation. An EMIT overestimation of 16.7 +/- 22.5% also was obtained for the MPA areas under the curve calculated using the trapezoidal rule.