About 5% of the Swiss forest surface should be protected as natural forest reserves or as old-growth islands until 2030. This goal, which has been formulated by the Federal Office for the Environment and by the cantons, aims to promote the natural development of the forest and to conserve saproxylic species. Beside large-sized natural forest reserves, smaller old-growth islands may also play an important role. A bootstrapping method has been developed to define the minimal size of old-growth islands based on a field inventory in high forests without logging since at least 30 years. Three different indicators for habitat structures have been used to establish the ecological potential of old-growth islands: the total number of habitat structures, the amount of deadwood and the number of woodpecker trees. The threshold has been defined as the average of each indicator minus the standard error for different forest types. The surface of an island is determined at the point where 75% of the modeled surfaces are above the threshold, which guarantees a certain ecological potential. This surface reaches 0.7 ha for floodplain forests, 0.9 ha for beech and fir-beech forests and 1.2 ha for silver spruce-fir forests. The computed values are minimal rather than optimal surfaces for old-growth islands. With an increase of the surface, sustainability, quantity and quality of the habitats will be improved. Therefore, the size of old-growth islands should be larger whenever the local circumstances are favorable.