Smoking is a risk factor for periodontitis but there are conflicting reports about the relationship between the severity of periodontitis and smoking behaviour, in part because self-reported smoking status may be inaccurate. Cotinine, a major metabolite of nicotine with a longer half-life (17 h versus 30 min), may be a more useful biochemical marker of smoking status. Smoking behaviour, plasma cotinine levels, and measures of periodontitis severity in 135 adults with moderate-advanced periodontitis were studied. Smokers had comparable periodontitis at a younger age than non-smokers. Smoking, as measured by cigarettes smoked per day and plasma cotinine levels, was significantly related to the severity of periodontitis.