The usefulness of mycophenolate mofetil (MMF) levels in stable kidney transplant patients is not well known. We measured MMF trough levels in 137 adult kidney recipients with more than 1 year of stable graft function. The MMF dose was adjusted according to hematological or gastrointestinal toxicity, it was 500 mg in 22 (16%) patients; 750 mg in 22 (16%); 1000 mg in 69 (50.5%); 1500 mg in 15 (11%); and 2000 mg in 9 (6.5%). We analyzed the total dose, virgule dose/kg, and MMF levels in relation to efficacy parameters (creatinine, proteinuria) and hematological toxicity (erythrocytes, leukocytes, and platelets) at the time of MMF level determinations and 3 months thereafter. Statistical analyses were performed with SSPS 12.0, including sensitivity and specificity analyses by ROC. Mean MMF levels were 3.68 mg/L (Pc25, 1.6-Pc75, 4.4 mg/L) with significant differences according to dose (P < .001). Trough MMF levels did not have discriminatory capacity in the area under the ROC for anemia, renal failure, or proteinuria at the time of determination or 3 months later. The percentage of patients without proteinuria was high among patients with MMF levels between 1.6 and 4.4 mg/L. The MMF levels were low in patients who had a major increase in creatinine (1.6 vs 3.8 mg/L, P < .05). In stable renal transplant patients the levels of MMF were related to the administered dose, and they are higher than those previously described in patients with less than a year follow-up with a functioning kidney. They did not have discriminatory value at the time of determination or 3 months later. Nevertheless, low MMF levels could help recognize patients at risk of developing chronic nephropathy.