Diameter structures, diameter–age relationships, and spatial pattern of canopy and understory layers in seven stands representative of mature lodgepole pine (PinusconfortaDougl. ex Loud. ssp.latifolia) forests on gentle (≤5°) slopes in the Taylor Park Basin of Colorado were documented. All stands originated following crown fires 120–140 years ago, during the period of initial Euro-American presence. Two distinct structures were evident among these stands. Four stands had closed canopies (cover >70%), relatively high basal areas and tree densities, low sapling and large seedling densities, numerous standing dead trees, and even-aged diameter distributions. The other three stands had more open canopies (cover <65%), lower basal areas and tree densities, higher densities of saplings and large seedlings, and more even representation across diameter classes. Regeneration tended to occur in small patches in both types of stands. DBH showed a moderate to strong relationship to age in all stands, although suppression of bole-diameter growth was more evident in the closed stands. Structure of the closed stands exemplifies forests that have developed rapidly after canopy fire. Structure of the open stands suggests either low stocking densities following stand-originating crown fires or repetitive surface fires, perhaps reinforced by dwarf mistletoe (ArceuthobiumamericanumNutt. ex Engelm.) infestation, that have allowed more or less continuous recruitment over the past 120–140 years.