Objective. To evaluate the conditions of the oral environment of different sectors of the mouth in patients before and during orthodontic treatment. Methods. A prospective study in 46 patients with fixed orthodontic treatment. Clinical and microbiological parameters of saliva and bacterial plaque were collected before treatment (t0) and at 45 (t1), 90 (t2), and 135 days (t3) after the start of treatment. Results. Before treatment, high values of ‘decayed’ component of the DMF-T and regular levels of the OHIs were registered. Fifty-seven percent of patients showed lower dental crowding. During treatment, the orthodontic plaque index OPI decreased with respect to OHIs at t0, although it maintained undesirable values for oral health (p=0.001). The buffering capacity decreased at t1 and t2 but recovered to initial values at t3 (p=0.001). Streptococcus mutans, Candida albicans, and Lactob...