Pioneer Mothers' Memorial Forest is a unique largely undisturbed, mixed mesophytic hardwood forest in south-central Indiana, USA. We report on 26 years of change in community composition and structure; plots were previously sampled in 1978 and 1989. In 2004, we re-measured trees ≥ 5 cm in diameter for 153 permanent plots, as well as downed woody debris, and standing dead trees. In 1978, density was 718 stems ha⁻¹ and basal area 26.2 m² ha⁻¹, but by 2004 density had decreased to 493 stems ha⁻¹ with an increase in basal area to 31.4 m² ha⁻¹. Large density decreases were attributed to Acer saccharum, Sassafras albidum, Juniperus virginiana, and Cornus florida mortality, while basal area increased largely because of Liriodendron tulipifera and A. saccharum growth. The stand exhibited a reverse-J diameter distribution, but density in the lower diameter classes, < 15 cm, greatly decreased over time. The overstory (trees ≥ 25 cm) was dominated by A. saccharum, L. tulipifera, Quercus spp., and Fagus grandifolia. The understory was dominated by A. saccharum, which also accounted for > 60% of the ingrowth over the observed period. Diversity exhibited a significant decline between 1978 and 1989 and a negative trend overall. Annual stand mortality averaged 2.2% since 1978, with a U-shaped distribution of high mortality rates in the smallest and largest diameter classes. Most plots (62%) experienced multiple canopy tree deaths, but were not sufficiently large to allow for the ingrowth of Quercus spp. Very few members of shade-intolerant species that dominated the canopy were able to attain canopy status, excepting L. tulipifera stems characterized by good growth rates (mean 7 mm yr⁻¹). Down woody debris and standing dead trees were moderately abundant in varying states of decay across all diameter classes. Without any major disturbance or understory disturbance, shade-intolerant species, notably Quercus and Carya spp., will likely be very limited in the stand, and the forest canopy will eventually be dominated by A. saccharum and F. grandifolia. This study highlights the importance of long-term studies of forest species composition and structure to understand forest ecosystem dynamics.