Objective: To evaluate the association between the presence of periodontal pathogenic bacteria in saliva and halitosis in mouth air. Design: Cross-sectional microbiological and clinical oral examination of adult patients. Subjects: 101 adult patients (25 males, 76 females) who attended the Preventive Dentistry and Breath Odour Clinic of Kyushu Dental College. Their average age was 50.0+/-13.5 years old (mean+/-SD). Setting: The subjects were classified into three groups: halitosis subjects with a probing depth (PD) greater than or equal to 4mm (P group), halitosis subjects without PD greater than or equal to 4mm (H group), and non-halitosis subjects without PD greater than or equal to 4mm (C group). Methods: All subjects received a periodontal examination. Volatile sulphur compounds (VSC: hydrogen sulphide and methyl mercaptan) were measured using gas chromatography. The presence of Bacteroides forsythus, Porphyromonas gingivalis, Actinobacillus actinomycetemcomitans and Prevotella intermedia in the saliva was detected by polymerase chain reaction. Results and conclusion: The presence of B. forsythus, P. gingivalis and P. intermedia influenced the production of VSC. Specifically, the presence of B. forsythus in subjects with periodontitis was strongly correlated to the concentration of VSC in mouth air.