Purpose: The purpose of this study was to evaluate a volatile sulfur compounds (VSCs) monitor's ability to assess oral malodour in patients with and without periodontal disease in comparison with the assessment by gas chromatography (GC) or organoleptic testing. Materials and Methods: Ninety-nine patients' mouth air was measured by GC, a VSCs monitor (BreathtronTM) and organoleptic test. Patients who had a periodontal pocket depth of 4 mm and more for at least two tooth surfaces were assigned to the periodontal disease group. Results: Total VSCs value by the BreathtronTM was higher in periodontal disease group than that in non-periodontal disease group, and it showed statistically significant correlations with specific VSCs gases from GC and with the organoleptic measurement. The BreathtronTM had high sensitivity in both groups. Conclusion: The BreathtronTM can be a reliable instrument for the diagnosis of halitosis. However, the BreathtronTM should be used properly for measuring VSCs that are related to periodontal disease. Keywords: Oral malodour assessment, volatile sulfur compounds (VSC)