Associations between oral malodor, measures of periodontal disease, and trypsin-like activity of periodontal pathogens on tongue and teeth were examined in 127 subjects. Volatile sulphur compound (VSC) measurements were made with a portable sulphide monitor; oral malodor was also estimated by organoleptic methods. Measurements repeated one week apart indicated that steady-state VSC levels (r = 0.72; P = 0.0001) and peak VSC levels (r = 0.63; P = 0.0001) were reproducible but these r values were not significantly different (P > 0.1). There was a significant correlation between tongue odor and peak VSC levels (r = 0.40; P = 0.0001) and between tongue odor and whole mouth organoleptic measures (r = 0.55; P = 0.0001). To study the effect of reducing microbial colonization on oral malodor, chlorhexidine gluconate (0.2%) rinsing was prescribed for 7 days. Reductions of VSC levels were significant for both peak (37%) and steady-state (41%) data (P = 0.0001). Anaerobic periodontal pathogens on the tongue estimated by the proportions of positive BANA tests were reduced 19% (P = 0.001) and this was concomitant with a 40% (P = 0.0001) decrease in organoleptic measurement of the tongue dorsum. Mean pH measurements of the tongue dorsum showed large reductions from 6.9 initially to 6.3 post-treatment (P = 0.0001). Subjects were divided into periodontitis/no periodontitis based on periodontal inflammation and probing depth (> or = 5 mm). Of the 37 subjects with periodontitis, 23 had oral malodor whereas 52 out of 90 periodontally healthy subjects exhibited malodor. Chi square analysis comparing halitosis in subjects with and without periodontitis showed no statistically significant association (chi 2 = 0.208; P 0.65) between these two factors although the intensity of malodor as based on VSC concentration in periodontally healthy subjects was 19% less (mean = 111 ppb) than in subjects with periodontitis (mean = 136 ppb). The odds ratio was 1.2, indicating that oral malodor was not associated with periodontitis. These data indicate that a large proportion of individuals with oral malodor are periodontally healthy and that the mucosal surface of the tongue is a major site of oral malodor production.