The purpose of this clinical and microbiological study was to evaluate longitudinally the changes occurring in the subgingival microbiota in children following the placement of orthodontic bands in the absence of a prophylactic oral hygiene program. A total of 12 children in the age between 10 and 15 years were selected for the study. The experimental group (E) consisted of 6 subjects scheduled for orthodontic treatment including the placement of fixed appliances. They were seen 1 week before and just prior to the placement of orthodontic bands. The control group (C) involved 6 children in the maintenance phase of orthodontic therapy in which removable retainers were used. All subjects were examined at 3-5 week intervals for a period of 4 months. At each examination, microbiological subgingival plaque samples were collected by means of sterile paper points. Plaque and gingival index scores as well as pocket probing depth at the site of sampling were determined. The microbiological samples were processed using continuous anaerobic culturing techniques and were plated on non-selective and selective media. Differential counts and biochemical characterization of isolates were performed according to the methods described by Kornman and Loesche. Following tooth-banding, an increase in pocket probing depth was observed, while the P1I and GI scores remained unaffected. A statistically significant increase from baseline values (p less than 0.05) was found for the %s of black-pigmented bacteroides, the B. intermedius and A. odontolyticus species, concomitantly with a decrease of the anaerobe/facultative bacteria ratio in the experimental, but not the control sites.(ABSTRACT TRUNCATED AT 250 WORDS)