5 patients in maintenance, 1-3 years after periodontal therapy who showed sites with reinfected pockets and bleeding despite regular recall visits were selected. Darkfield microscopy from 3 sites in each patient showed an average of 41% spirochetes and 21% motile rods. Probing depths ranged from 7 to 9 mm and loss of clinical attachment from 6 to 13 mm in these sites. The patients were given 3 times 250 mg/day of metronidazole (Flagyl) for 10 days. Darkfield microscopy and microbiological cultures of the subgingival plaque were performed twice prior to the study, at the end of the medication and after 3 weeks, 3 and 6 months. The plaque and gingival indices, probing depth and loss of clinical attachment were recorded. During the medication and at 3 and 6 months, the teeth were scaled and root planed. The samples were obtained with 3 paper points and cultured anaerobically in the glove box on non-selective and selective media and representative bacterial colonies identified by aerobic growth, gram stain and rapid biochemical tests. Presumptive pathogenic micro-organisms including Bacteroides were identified. The % of spirochetes, motile rods and non-motile bacteria were enumerated by darkfield microscopy. The clinical results show that administration of metronidazole and repeated root planing significantly decreased gingival inflammation, probing depth and loss of clinical attachment in reinfected sites. After treatment, these sites harbored significantly less spirochetes and more non-motile bacteria, while motile rods tended to return to baseline levels with time. The combined antibiotic and mechanical therapy resulted in a statistically significant decrease of gram-negative rods, Fusobacteria and Bacteroides gingivalis over 6 months.(ABSTRACT TRUNCATED AT 250 WORDS)