Gingival biopsies were taken from 13 renal transplant patients (mean age 26.5 yr), 11 of whom exhibited cyclosporin A (CsA)-induced gingival overgrowth. Control material was obtained from seven volunteers (mean age 28 yr). Gingival tissue components were analyzed by quantitative microscopy (stereology) on 5-microns-thick sections of interdental papillae. The volume density (Vv) of different tissue components and the surface density of epithelial ridges were calculated by conventional point and intersection counting. The study showed that the volume density of oral epithelium and the surface density of the epithelial ridges in the CsA-induced gingival overgrowth were significantly increased compared to normal gingival tissue. The connective tissue of the lesion exhibited a significant increase in volume density of cells, blood vessels and non-collagenous matrix with a corresponding decrease in the collagenous matrix. These results indicate that CsA-induced gingival overgrowth represents a tissue with an altered composition characterized by increased thickness of oral epithelium and relatively increased amount of cells, vessels, non-collagenous matrix and decreased collagenous matrix in the connective tissue.