Incidences of mortality without any clinical signs occurred in growing chicks in a broiler flock. Five 17-day-old dead broilers with gizzard erosions were investigated pathologically. Macroscopically, the gizzards were found to be dilated with bloody fluids. The koilin layer of the gizzards showed multifocal black patches (erosions). Histologically, there was necrosis of the koilin layer, degeneration and depletion of the gizzard glandular epithelium with intranuclear inclusion bodies, and hyperplasia of the macrophages in the lamina propria. An immunohistochemical analysis revealed intranuclear inclusion bodies in the degenerating gizzard glandular epithelial cells that stained positively for group I avian adenovirus antigen. Ultrastructurally, numerous viral particles (an average of 72 nm in diameter, from 63 to 88 nm) were in the intranuclear inclusions of glandular epithelium. This study suggests that the gizzard erosions may have been caused by group I avian adenovirus. In addition, the degeneration and depletion of gizzard glands secreting koilin substances, by adenovirus, may induce a reduction of koilin substances, which may then cause gizzard erosion (focal loss of the koilin layer).