The aim of this study was to evaluate the prevalence of furcation-involved molars in a patient sample referred for periodontal treatment. A total of 222 patients aged, 14-73 years (mean age 45 years), and with signs of destructive periodontal disease in at least 2 quadrants of the dentition were included in the study. The clinical examination involved assessments of oral hygiene status, gingival conditions, probing pocket depth and presence/degree of furcation involvement. In addition, a full mouth intraoral radiographic examination was performed. The results revealed that 4% of the patients presented with all 12 molars, while 3% had lost all molars. 52% of the individuals had at least 8 molars; 95% of subjects < 30 years of age and 19% in the age 60+ years. In patients aged > or = 40 years, every 2nd molar showed advanced periodontal destruction in the furcation area. The prevalence of furcation involved molars was higher in the maxilla than in the mandible. From the age of 30 years, about 50% of the 1st and 2nd molars in the maxilla showed at least 1 furcation site with deep involvement, while in the mandible a similar prevalence was observed first after the age of 40 years. The highest frequency of furcation involvement was found at the distal site of the maxillary 1st molar (53%), and the mesial aspects of the maxillary 2nd molar showed the lowest frequency (20%). Furcation sites with a probeable trunk region were observed at a frequency of 17-22% at the various tooth sites. It was concluded that tooth morphology may be an important factor that accounts for the variability in prevalence of molar furcation involvement.