Periodontal bone destruction adjacent to furcations of molar roots has been considered to be unfavourable for the prognosis of the involved teeth. Several alternative modes of treatment have been reported. The objective of this study was to find out how important the furcation problem is. A study was conducted on the prevalence, degree and development of bone destruction between the roots of furcated mandibular molars in a sample of 221 individuals observed over a period of 13 years. The state of the interradicular bone septa was assessed from orthopantomographs and bite-wing radiographs. Radiographs were available from 1965, 1971, 1974 and 1978. The degree of bone destruction was measured in relation to the root length. About 90% of remaining molars (in 1965) were furcated. The frequency of bone destruction increased from 18% in 1965 to 32% in 1978. Only 1.1-2.7% of the molars had bone loss affecting more than 50% of the distance vertex to apex. About 9% of the furcated molars were lost during the observation period. However, the percentage of teeth lost due to progressive furcation involvement during the 13 years was estimated to be only 2.5%.