The specific activities of the succinate dehydrogenase-coenzyme Q(10) reductase in gingival tissue from patients with periodontal disease have been compared with the corresponding specific activities of normal human periodontal tissue. The gingival biopsies from patients having diseased periodontal tissue showed a deficiency of coenzyme Q(10), in contrast to those of the normal periodontal tissue which showed no deficiency. The presence or absence of a deficiency of coenzyme Q(10) in the succinate-coenzyme Q(10) enzyme system is appraised by determining the specific activity in the absence and again in the presence of exogenous coenzyme Q(3). An increase in specific activity of this mitochondrial enzyme system in the presence of exogenous coenzyme Q(3) reflects the mitochondrial deficiency of coenzyme Q(10). Such increases ranged from 38-120% and averaged 81% for the individuals with periodontal disease, and were highly significant statistically. These data correlate with clinical studies in Japan that have indicated a therapeutic benefit of the administration of coenzyme Q(7) to many patients with severe and destructive periodontal disease and with the benefit of administration of hexahydrocoenzyme Q(4) to one such patient in the current work.