We examined tree mortality agents in pristine old Norway spruce (Picea abies (L.) Karst.) forests in northern Finland and northwestern Russia. The data was collected on nine 40 m 400 m transects. The primary mortality agents of recently dead trees were recorded and their frequencies were calculated. The pattern of tree growth prior to death was studied based on increment core samples and compared with the growth of healthy dominant trees. Of all recently dead trees, 72% could be associated with a primary mortality agent. In both study areas the most common primary mortality agent was a Coniophora (Mérat) DC. -genus fungi, which was found on average in 33% of trees sampled. The fungi Phellinus chrysoloma (Fr.) Don and Onnia leporina (Fr.) H. Jahn as mortality agents were more common in the Finnish area compared to the Russian area. Analysis on the growth patterns indicated weak differences between different pathogens’ influence on prior-to-death growth of trees, so that fungi rotting the whole tree decreased tree growth more rapidly than fungi rotting only the heart wood. The results demonstrated that in old Norway spruce forests of northern Fennoscandia the most common primary tree mortality agents were wood rotting fungi, which weaken the mechanical stability of tree stems until they fall due to snow or wind, which should be considered only as secondary mortality agents. It is evident that tree death in pristine forest typically results from a long-lasting process involving both biotic and abiotic factors.