AbstractThe purpose of this study was to evaluate the use of Ornidazole as an adjunct to root planing in the therapy of patients suffering from recurrent periodontal disease. In 10 individuals who had previously been treated with scaling, root planing and periodontal surgery and who had followed a regular maintenance program including recall visits every 3‐5 months for 1–7 years, 2 sites with recurrent periodontitis and 1 shallow site were selected. Reinfected sites had a record of losing clinical attachment of more than 3 mm since the completion of initial therapy, were bleeding upon probing and had a mean pocket probing depth of 7.85 ± 1.31 mm. They had been reinstrumented several times by a registered dental hygienist, when clinical signs of recurrence of disease had appeared and the root surfaces were judged to be smooth and free of deposits. Clinical parameters were recorded and microbial samples were collected twice prior to retreatment. Then, 500 mg Ornidazole, to be taken twice a day for 10 days, was administered, and the whole dentition was thoroughly scaled and root planed. At day 10 as well as 2, 5, 8 and 11 months thereafter, samples were again obtained. At baseline, reinfected sites showed over 20% spirochetes, over 20% motile rods and over 9% fusiform organisms in darkfield preparations of subgingival plaque samples. Culturally, over 1/10 of organisms were identified as black pigmentingBacteroidesand in 18% of all baseline samples collected,B. gingivaliswas found. Immediately following therapy, total counts of bacteria were reduced to approximately 5% of the baseline counts and more than half of the bacteria were characterized as coccoid cells; Gram‐negative anaerobic rods were reduced to less than 3%. Among them, black ‐pigmentingBacteroides, were reduced below 1%.B. gingivaliscould no longer be detected. In the first weeks following therapy, most microbiological parameters showed a tendency to increase, but after some time, the microbiological situation appeared to reach a steady state and a statistically significant long‐term reduction of Gram‐negative rods, includingBacteroidesssp. persisted. These findings were paralleled by a significant decrease of pocket probing depth, no further loss of attachment and a decrease in bleeding tendency. These findings indicate that Ornidazole has an additional, beneficial clinical and microbiological effect when used as adjunct to mechanical re‐treatment of recurrent