The specific activities of the succinate dehydrogenase-coenzyme Q10 reductase in mitochondria were determined for patients from a normal periodontal practice. The criteria for selection were patients having a bone score of 1.0-4.0 and a pocket depth of 2.5-5.2 mm. All 29 patients showed a deficiency of 20-63% of CoQ10-enzyme activity in gingival biopsies. The mean value was elevated (P less than 0.001) over that of controls. For corresponding blood samples, 24/28 (86%) showed deficiencies of 20-66% and a higher (P less than 0.001) mean value than that of controls. Periodontal patients frequently have significant gingival and leucocytic deficiencies of CoQ10. The leucocytic deficiency indicates a systemic nutritional imbalance and is not likely caused by neglected oral hygiene. A gingival deficiency could predispose this tissue to periodontitis and this disease could even augment the deficiency. These results support previously suggested adjunctive use of CoQ10 with oral hygiene for improved treatment presumably through bioenergetics.