To compare the subgingival temperature in healthy and diseased sites of gingiva in smokers and non-smokers using a modified digital thermometer. To also find whether subgingival temperature measurements can be used as reliable indicator of periodontal disease activity.Participants for this study comprised 50 males, 25 smokers and 25 non-smokers with mild to moderate periodontitis, aged 25-40 years and had four maxillary incisor teeth (12, 11, 21, 22) intact. For each participant, six sites around each tooth were examined. Hence, a total of 24 sites for each participant were examined. The clinical parameters used were probing pocket depth and gingival bleeding index.Sublingual and subgingival temperature was found to be warmer in smokers compared to non-smokers. Subgingival temperature was more in diseased sites than healthy sites in both smokers and non-smokers. The mean temperature differential of diseased sites in smokers was more compared to non-smokers. Mean probing pocket depth was higher in smokers, but bleeding sites were less.Subgingival temperature measurement was found to be a good indicator of periodontal disease activity.