In regions where primeval forests have vanished it is unclear whether forest protection can sustain specialized old-forest biota, and over what time scale. We report on population expansion of an old-growth specific fungus of European conservation concern, Amylocystis lapponica, in the forest reserve network of Estonia. This conspicuous species was known for 40 years from only single records in one old-growth forest and was categorized nationally as Critically Endangered. During the last 10 years A. lapponica has expanded over the eastern half of the country, with nine subpopulations, in 12 localities, now known, all in long-protected old-growth forests and several > 50 km apart. In most of the new localities historical absence of the species can be reliably assessed based on earlier surveys. The historical remnant subpopulation has also increased. The population size (c. 100 mature individuals) in Estonia indicates the species should be recategorized nationally as Endangered. This success story suggests that more than 50 years of non-intervention may be needed even in large old-forest reserves for old-growth specialist species to recover.