The specific activities of both the succinate dehydrogenase-coenzyme Q(10) reductase and the DPNH-cytochrome c reductase [NADH:(acceptor)oxidoreductase, EC 1.6.99.3] were determined in mitochondria from 40 diseased gingival biopsies from patients with periodontal disease and from 24 control biopsies from nondiseased areas (clinically evaluated) of gingival tissues from the same mouths of the patients from whom the diseased gingival tissues were taken. The control tissue was taken during normal surgical procedures, such as for gingival recontouring and tuberosity removal. The diseased gingival biopsies showed a mean specific activity for the succinate dehydrogenase-coenzyme Q(10) reductase which was higher (P < 0.02) than that of the control biopsies, and which increased (P < 0.01) when the assays utilized exogenous coenzyme Q(3), and corresponded to an average deficiency of coenzyme Q(10)-enzyme activity of 35%. About 60% of the 40 diseased gingival tissues showed a deficiency of coenzyme Q(10) at its site in this succinate-coenzyme Q(10) enzyme. Of the 24 control tissues, 20% showed deficiencies of coenzyme Q(10). As a group, the control tissues showed no deficiency of coenzyme Q(10). No deficiency of coenzyme Q(10) at its site in DPNH-cytochrome c reductase was observed for either the control or diseased gingival tissues, as groups or individually.