The aim of this study was to identify factors affecting the pharmacokinetics of mycophenolic acid (MPA) and its 7-O-glucuronide (MPAG) in systemic lupus erythematosus (SLE) patients. Thirty-one SLE patients in remission maintenance phase treated with mycophenolate mofetil (median 1500 mg/d) and prednisolone and followed-up for up to 56 months (median 13 months) were enrolled. Creatinine clearance and metal medication were significant predictors accounting for interindividual variability in the dose-normalized predose plasma concentration (C₀) of MPA (adjusted R²=0.305, p=0.01) in a multivariate analysis. Dose-normalized MPAG C₀ was significantly correlated with only creatinine clearance (adjusted R²=0.135, p=0.03). The free fraction of MPA was significantly correlated with only serum albumin (adjusted R²=0.700, p<0.01). The free fraction of MPAG was significantly correlated with serum albumin, metal medication, and age (adjusted R²=0.598, p=0.02). In conclusion, renal function and co-administered metal influenced the pharmacokinetics of MPA and MPAG in SLE patients in remission maintenance phase.