The purposes of this study were to determine if: 1) an association exists between cigarette smoking and signs of periodontal disease after controlling for the confounding variables of age, sex, plaque, and calculus; 2) the prevalence of 5 bacteria commonly associated with periodontal disease differs between smokers and non-smokers; and 3) the presence of any of these bacteria or smoking are associated with a mean proximal posterior probing depth > or = 3.5 mm. Plaque, calculus, gingivitis, and probing depth were measured at the proximal surfaces of all teeth in one randomly selected posterior dental sextant in 615 adults. Subgingival plaque was sampled from the same sites and assayed for the presence of Porphyromonas gingivalis, Actinobacillus actinomycetemcomitans, Prevotella intermedia, Eikenella corrodens, and Fusobacterium nucleatum. A subsample of non-smokers (n = 126), who were similar to smokers (n = 63) with respect to age, sex, plaque, and calculus, was randomly drawn from the original sample. These two groups were then compared on the basis of clinical and microbial parameters. The results indicated that the odds of having a mean probing depth > or = 3.5 mm were 5 times greater for smokers than the non-smoker subsample (odds ratio = 5.3; 95% CI = 2.0 to 13.8). No statistically significant difference in the prevalence of any of the bacteria was found between smokers and the non-smoker subsample. Based on logistic regression analyses of each of the 5 bacteria and smoking, mean probing depth > or = 3.5 mm was significantly associated with the presence of A. actinomycetemcomitans, P. intermedia, E. corrodens, and smoking (P < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)