Old-growth forests are spontaneously developed forest ecosystems without direct human influence in which only natural processes take place. In this study we analyzed the structural sustainability of beech-fir-spruce old-growth forests on dolomite and limestone in the Bosnian Dinaric Mountains. The field work was carried out on permanent experimental plots of 1.0 hectare in size. Thereby, the diameters (d1.30) and the height (h) of all trees within the plots were measured. Based on the available literature, we hypothesized that the structure of old-growth forests provides sustainability through tree-size demographic equilibrium. Thus, the data collected were used to test possible differences between the actual and the theoretically balanced structure in the studied old-growth forests. Statistically significant difference in the actual structure between the two old-growth forests on limestone and dolomite was determined. However, both of them exhibited sustainable diameter distributions. These results point to the importance of preserving old-growth forests for future research as they exemplify the tree-size demographic sustainability and can thus serve as an appropriate reference to managed forests. Concretely, certain structural attributes from old-growth forests could be embedded into the management objectives for increased resilience of managed forests.