Mouth breathing is an important etiological factor of chronic gingivitis and marginal periodontitis as well as dental plaque and calculus. The purpose of this study is to demonstrate (a) the frequency of clinical signs of mouth breathing in school children (b) correlation between 8 individual signs (c) relationship between prevelance of gingivitis (PMA Index) and clinical signs and symptoms of mouth breathing (d) relationship between oral hygiene status (OHI) and each clinical signs. The materials were examined, 1) lipseal insufficiency 2) lipdrying 3) over jet 4) open bite 5) mouth breathing line 6) tension ridge of palatal and lingual surface 7) hypertrophy of tonsils 8) lack of uvular reflex. As the result of this investigation, the followings were concluded. 1) Lip-seal insufficiency, mouth breathing line, hypertrophy of tonsils and lack of uvular reflex were found in more than 50% of children. 2) Lip-seal insufficiency, lip drying, over jet, mouth breathing line and tension ridge were related each other respectively. 3) Excepting hypertrophy of tonsils and lack of uvular reflex, all other signs were individually related to both of OHI and PMA.