We prospectively studied the impact of peritoneal dialysis (PD) on the pharmacokinetics of mycophenolic acid (MPA) in five patients following renal transplantation. Three patients had a glomerular filtration rate (GFR) of less than 10 ml/min and two had a GFR of more than 40 ml/min. Pharmacokinetics of MPA and of its main metabolite, mycophenolic acid glucuronide (MPAG), were studied during two consecutive 12-h periods (with and without PD). After initiation of PD in patients with severe renal impairment (GFR < 10 ml/ min), MPA-area-under-the-concentration-curve (AUC) decreased up to 59% and MPAG-AUC decreased up to 26%. We did not observe any substantial changes in the MPA-AUC or MPAG-AUC of either patient with a GFR above 40 ml/min. Patients with a reduced GFR had much higher MPAG values than patients with a GFR above 40 ml/l; yet, we did not observe any differences in the MPA values. We found a significant inverse correlation between GFR and MPA-AUC (r = 0.81, P < 0.05) and between GFR and MPAG-AUC (r = 0.94, P < 0.01). While MPA was found only in traces in the peritoneal ultrafiltrate, the cumulative amount of MPAG removed by PD reached up to 2 g/ 12 h, representing 1.2 g of MPA. This is the first report describing a reduction in MPA-AUC and MPAG-AUC during PD. Further studies are needed to completely understand the pharmacokinetics of mycophenolate mofetil during PD.