The role of smoking as a risk factor for Periodontitis was assessed separately in diabetic and nondiabetic study groups. Subject listings stratified for age (19 to 40 years) and sex were obtained for subjects with insulin-dependent diabetes mellitus (IDDM) and nondiabetic subjects. For both the IDDM group (n = 132) and the nondiabetic group (n = 95), age and sex stratified samples were constructed by random selection of subjects from each subject listing. Patients were recruited by phone, examined, and their medical and dental histories obtained. Among nondiabetic subjects, the prevalence of Periodontitis was markedly higher among current smokers compared with never smokers (P < 0.005) in both the 19 to 30 year-old (46% vs. 12%) and 31 to 40 year-old groups (88% vs. 33%). The subject mean percent of sites with gingival pocket depth >4 mm was higher among current smokers than never smokers (P = 0.001) in the 19 to 30 (8.2% vs. 3.4%) and 31 to 40 (14.3% vs. 4.3%) age groups. The effects of smoking among IDDM subjects were similar to that observed in the nondiabetic population. There were no differences between current and never smokers in the proportion of sites positive for plaque. Attributable risk percents from prevalence data suggest that among nondiabetic subjects, a large proportion, perhaps as much as 51% of the Periodontitis in the 19 to 30 year old group and 32% of the Periodontitis in the 31 to 40 year old group, is associated with smoking. These findings suggest that smokers are a high risk group for Periodontitis, and that smoking may be the single most important environmental risk factor for Periodontitis. / Periodontol 1993; 64:16-23.