Woodpeckers are important primary excavators that produce nest-sites that are also used by several other cavity-nesting animals. However, the value of nest sites produced by different woodpecker species for other cavity-nesters is not generally known. We studied old Three-toed Woodpecker Picoides tridactylus cavities to examine their importance for other cavity-nesting forest bird species. Over a 31-year period (1987–2017), we annually monitored the occupancy of 655 cavities excavated by the Three-toed Woodpecker in 86 territories of the species in a 170-km² area in southern Finland. Eight bird species used old Three-toed Woodpecker cavities for breeding, and the occupancy of these cavities by all secondary users was 21.3%. The expected mean number of lifetime occupancies by secondary cavity-nesters for a single cavity was 2.03. This is equivalent to a mean of two cavities occupied per territory per year, if we assume that Three-toed Woodpeckers produce a new cavity every year. The actual numbers of annually occupied cavities by secondary cavity-nesters can be substantially lower especially in territories that are not constantly occupied by the Threetoed Woodpecker. Based on the use of old cavities and densities of secondary cavity-nesting bird species, we conclude that cavities made by Three-toed Woodpeckers are most significant for the species itself, which accounted for 23.3% of nests in old Three-toed Woodpecker cavities. When Three-toed Woodpeckers reused an old cavity rather than occupy a freshly excavated cavity, 93% of cavities selected were originally excavated by Three-toed Woodpeckers. In addition, old cavities are important for the Pygmy Owl Glaucidium passerinum — up to 26–31% of breeding attempts were in old Three-toed Woodpecker cavities. The importance of old cavities for other hole-nesting bird species like the Pied Flycatcher Ficedula hypoleuca or the Great Tit Parus major was rather minor considering their densities within woodpecker territories.