Previous studies have shown that a 3-day exposure of the pocket flora to the sustained release of chlorhexidine significantly reduced the relative numbers of spirochetes and motile rods in periodontal pockets to negligible amounts. By 14 days post-treatment, their numbers had returned to pre-treatment levels. The present study extended the exposure time of the pocket flora to the sustained release of chlorhexidine in an attempt to prolong the suppression of the microbial flora for a clinically significant period of time. Clinical parameters were also studied. Sustained release devices (SRD) were inserted into 13 pockets from 8 patients. Pocket depth ranged between 5 and 8 mm. The SRD's were replaced every 3 days to give a total exposure of 9 days. Plaque index (PlI), bleeding on probing and pocket depth were measured, and bacterial samples taken for dark field microscopy and anaerobic culture. There was a marked decrease in the relative proportions of spirochetes and motile rods and the total anaerobic count post-treatment. Pocket depth was reduced in all 13 pockets. These results indicate that a prolonged exposure to chlorhexidine suppresses the pocket flora to negligible amounts and reduces pocket depth for up to 11 weeks post-treatment.