The aim of this study was to investigate the relationship between cigarette smoking and furcation involvement in molar teeth. A consecutive group of 50 smokers were recruited from referrals to a periodontal clinic and age and gender matched with never smokers. Smokers consumed an average of 18.0 (SD 6.7) cigarettes per day and had smoked for 20.7 (SD 6.5) years. Radiographs of all molar teeth were assessed 2X by an examiner blinded to the smoking status. Smokers had slightly fewer molar teeth 6.7 (SD 2.6) than the never smokers, 7.3 (SD 2.3), t=1.2, P=0.22. More smokers (72%) had evidence of furcation involvement than never smokers (36%), chi(2)=13.0, P=0.0003. The odds ratio for a smoker having 1 molar with furcation involvement was 4.6 (c.i. 2 - 10.6). Smokers had more molars with furcation involvement 1.94 (SD 1.7) compared with never smokers, 0.94 (SD 1.4), t=3.1, P=0.003. It is concluded that cigarette smoking is associated with a greater expression of molar furcation involvement in periodontitis affected subjects.