The aim of the present study has been the evaluation of the effectiveness of oral irrigation with or without toothbrushing and dental flossing, in individuals treated with fixed orthodontic appliances, on controlling the development of dental plaque and, hence, of gingivitis. Eight individuals with a good general and oral status have been chosen. Before the experimental period, they received instructions about oral hygiene with toothbrushing and dental flossing and then they have been monitored to verify they were doing well. At the time T0, the upper tooth have been banded, three Periodontal Indexes (Plaque Index according to Silness and Loe, Modified Gingival Index according to Lobene and Gingival Bleeding Index according to Ainamo and Bay) registered and subgingival plaque samples from the premolars' gingival sulcus collected in order to point out the total anaerobes bacterial counts, the rates of motile bacteria, spirochetes, Gram positive and Gram negative cocci and bacteria, by means of optical and dark field microscopy and of cultural methods. For their oral hygiene, the patients had to use, in the right side toothbrush and dental floss (Control 1), in the left side the oral irrigator alone (Test 1). One month later (time T1), the lower teeth have been banded, too. In the right side the patients had to use toothbrush and dental floss (Control 2), while in the left one they used the same devices as Control 2 plus the oral irrigator (Test 2). At the time T1 Periodontal Indexes and Microbiological analyses have been extended to all the four quadrants.(ABSTRACT TRUNCATED AT 250 WORDS)