Background&Aims: Despite evidence for therapeutic efficacy with ursodeoxycholic acid (UDCA) in primary biliary cirrhosis (PBC), only 30-50%of patients achieve complete biochemical remission within 1 year of therapy. Mycophenolate mofetil (MMF) is an immunosuppressive medication that inhibits T and B lymphocyte proliferation. The aim of this investigation was to determine the safety and estimated efficacy of MMF in patients with PBC. Methods: Twenty-five patients with incomplete responses to UDCA (defined as persistent elevation of serum alkaline phosphatase ≥2 times the upper limit of normal) received MMF 1 g daily to a maximum of 3 g daily with UDCA (13-15 mg/kg per day) for 1 year. Liver biochemistries were determined at 3-month intervals with Mayo Risk Score calculated at baseline and end of therapy. Results: Nineteen (76%) patients completed 1 year of therapy. Despite improvements in serum alkaline phosphatase (920 ±308 vs. 709 ±242 IU/L, P = 0.001) and AST (65 ±31 vs. 51 ±19 IU/L, P = 0.007) levels, these findings were clinically insignificant. Exploratory analysis revealed a strong correlation between advanced PBC defined by higher Mayo Risk Score and reduction in serum alkaline phosphatase levels (r = -0.74, P = 0.006). Six patients (24%) did not complete therapy; adverse drug events were responsible for study withdrawal in 3 individuals. Adverse reactions that resolved spontaneously or by dose reduction occurred in 13 patients. Conclusions: MMF is not associated with important clinical benefits in PBC based on the results of this pilot investigation. Background & Aims: Despite evidence for therapeutic efficacy with ursodeoxycholic acid (UDCA) in primary biliary cirrhosis (PBC), only 30-50% of patients achieve complete biochemical remission within 1 year of therapy. Mycophenolate mofetil (MMF) is an immunosuppressive drug for inhibits T and B lymphocyte proliferation. The aim of this investigation was to determine the safety and estimated efficacy of MMF in patients with PBC. Methods: Twenty-five patients with incomplete responses to UDCA (defined as persistent elevation of serum alkaline phosphatase ≥2 times the upper limit of normal) received MMF 1 g daily to a maximum of 3 g daily with UDCA (13-15 mg / kg per day) for 1 year. Liver biochemistries were determined at 3-month intervals with Mayo Risk Score calculated at baseline and end Despite improvements in serum alkaline phosphatase (920 ± 308 vs. 709 ± 242 IU / L, P = 0.001) and AST (65 ± 31 vs. 51 ± 19 IU / L, P = 0.007) levels, the findings were clinically insignificant. Exploratory analysis revealed revealed strong correlation between advanced PBC defined by higher Mayo Risk Score and reduction in serum alkaline phosphatase levels (r = -0.74, P = 0.006) adverse drug events were responsible for study withdrawal in 3 individuals. Adverse reactions that resolved spontaneously or by dose reduction occurred in 13 patients. Conclusions: MMF is not associated with important clinical benefits in PBC based on the results of this pilot investigation.