The effect of local applications of sustained-release delivery polymer containing chlorhexidine as an adjunct to mechanical plaque removal was studied in institutionalized children with Down's syndrome. Thirty children, ages 8-13, participated in the study. Clinical parameters (P1I, GI, papillary bleeding) were recorded and bacterial samples from selected permanent teeth were collected and processed before and following treatment. Following the registration of clinical findings and collection of plaque, all children had their teeth scaled and polished and were randomized into three treatment groups of ten subjects each: Subjects in group I had their teeth coated with ethyl cellulose containing chlorhexidine; those in group II had their teeth coated with placebo polymer and those in group III received no further treatment. Individual oral hygiene habits were not interfered with. The application of the solutions to the respective groups was done every 3 days for 21 days. Chlorhexidine treatment significantly reduced the PLI, GI total aerobic counts and S. viridans counts compared to no treatment. The placebo group showed similar effects to that of the chlorhexidine-treated group except for the total aerobic counts which were not affected. The results suggest that the use of chlorhexidine in a sustained-release dosage form applied to the tooth surfaces may prove useful in the control of plaque and its sequela in children with Down's syndrome.