. Long-term safety of treatment with hepatitis B virus (HBV) polymerase inhibitors is a concern. Adefovir dipivoxil (ADV) therapy has previously been associated with impairment of renal function. Limited data are available on the safety of combination therapy with nucleos(t)ide analogues and interferon alfa (IFNa). The aim of this analysis was to assess the renal function during combination therapy with peginterferon alfa-2a (PegIFNa-2a) plus ADV vs either drug alone in patients with hepatitis B/D co-infection. We performed a retrospective analysis of renal function data of patients treated in the Hep-Net/International Delta Hepatitis Intervention Trial 1(HIDIT-1-trial), a European multicenter study to investigate the efficacy of 48 weeks of therapy with PegIFNa-2a+ADV vs either drug alone in 90 patients with chronic hepatitis B/D co-infection. Glomerular filtration rates (GFR) were calculated by CockcroftGault (CG), abbreviated Modification of Diet in Renal Disease (MDRD) study and Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation. After 48 weeks of therapy GFR values were significantly lower in patients receiving adefovir-containing treatment vs PegIFNa-2a alone [mean difference 16.1 mL/min (CG) and 10.2 mL/min (MDRD), respectively, P < 0.05] while no differences were observed between patients receiving adefovir alone vs combination treatment. Twenty-four weeks after treatment GFR values did not differ between treatment arms. A decrease in GFR =20% was observed more often in patients during adefovir-containing treatment vs PegIFNa-2a alone (P < 0.05) which was confirmed by KaplanMeier analysis. Adefovir-containing but not PegIFNa-2a treatment was associated with a decrease in GFR values in about one-fifth of patients. Combination treatment of PegIFNa-2a+ADV in chronic hepatitis B/D co-infection did not lead to any further impairment of kidney function.