The normal bacterial flora in the nasal cavity will be different in various states of the nose and paranasal sinuses. The pH of the nasal cavity will be changed due to acute and chronic inflammation from neutral to acid or alkali side. Since Fabricant(1941) reported that normal pH range of nasal cavity was in 5.6-6.5, some authors have measured it, and they do not agree to his opinion. They said that the normal pH of nasal cavity was rather neutral than acid, and my data was also neutral in the range of 6.8-7.2. Park (1971), in Korea, reported that the average pH of the nasal cavity was 7.3, and he also measured the pH of nasal cavity in different pathologic state. Twodis(1936) and Buhrmester(1933), who investigated the relation between pH and bacterial flora, concluded that there was no bacterial growth in vitro when the pH of nasal cavity was below 6.5 due to lysozyme activity and this concept was supported by Fabricant. I measured the pH of nasal cavity from 30 patients who had chronic paranasal sinusitis and cultured the bacteria of nasal cavity at the same time and obtained some interesting data.