Patients admitted to the School of Dentistry, Stockholm, for treatment of chronic periodontal disease during the years 1980-82 were retrospectively investigated with respect to their smoking habits. The investigation was designed as a case control study and covered all patients 30, 40, or 50 yr of age upon admission, in all 155. As control served a random sample of the Stockholm population. The periodontal variables under scrutiny were frequency of periodontally diseased teeth, frequency of periodontally diseased sites (probing depth greater than 4 mm), gingival index, and plaque index. The overall occurrence rate of smokers in the sample of cases was 56%, which is significantly greater than the population at large. This held true for all three age cohorts and for men as well as women. The risk ratio was 2.5, indicating more prevalent disease among smokers. Further, significantly greater frequencies of periodontally involved teeth and diseased sites were found in smokers, indicating more severe disease among smokers. Gingival index and plaque index did not notably differ between smoking groups. The results suggest increased prevalence as well as severity in smokers. Smoking, therefore, should be considered a risk factor for chronic periodontal disease.