The histological appearance of the gingiva in children with Down's syndrome (DS) was studied with special reference to inflammatory involvement and innervation. A dense infiltration of inflammatory cells was seen in the propria of most of the DS patients, including a few polymorphonuclear leucocytes. A hyperplasia of the epithelium was also found. The innervation of the gingiva was studied using immunohistochemistry. Nerve fibers as well as nerve bundles immunoreactive to neurofilament (NF) were seen in the propria, while occasionally intraepithelial NF fibers were observed. Calcitonin gene-related peptide (CGRP)-immunoreactive fibers and fiber bundles were also visualized, but they were less abundant than NF fibers. The density of NF and CGRP fibers and fiber bundles was estimated by semiquantitative evaluation. A higher density of NF and CGRP immunoreactive structures was observed in the propria of DS patients compared to the control subjects, while no obvious alteration was seen in their distribution in the propria. In addition, sparsely distributed fibers immunoreactive to peptide histidine isoleucine amide (PHI) and vasoactive intestinal polypeptide (VIP) fibers as well as neuropeptide Y (NPY) and tyrosine hydroxylase (TH) were seen, mainly surrounding blood vessels. A few substance P (SP) fibers were also found, mostly close to the epithelium. No obvious differences of these sparsely distributed fibers were seen in the DS patients compared to controls. Thus, a profound inflammatory involvement of the gingiva of DS patients is seen concomitant with a hyperinnervation of the presumed sensory component of the gingival innervation. In contrast, no alterations were seen in the density of neuronal markers related to autonomic nerve fibers. The sensory hyperinnervation observed is probably not specifically related to DS, but may be due to a sprouting of afferent nerves induced by the inflammatory reaction. However, factors released from the sensory afferents could contribute to the gingival inflammation seen in DS.