This study examined the influence of acid cheese whey (ACW) addition on the nitrogen and phosphorus release in the co-digestion with sewage sludge (SS). The laboratory installation consisted of two semi-flow anaerobic digesters operating under mesophilic conditions. The concentrations of total nitrogen (TN), total phosphorus (TP), NH4+-N and PO43--P in the feedstock and the digestate were determined together with the appropriate release factors. The results indicate that the co-digestion of SS with ACW did not cause a significant increase in the concentration of biogenic elements both in the reactor feedstock and the digestate. Lower concentration of total nitrogen and total phosphorus was achieved in the digestate, both in whey and reference runs, probably due to partial retention in the digesters due to the precipitation processes.