This is a sequel to a previous study that showed that many molars with furcation involvement (F.I.) can survive in a state of health and can function efficiently and without pain for many years. This finding is at variance with the views of others who believe that the prognosis for teeth with F.I. is unfavorable. The conclusions of the present study are: (1) F.I. of molars was a common finding; it occurred much more frequently than anticipated. (2) F.I. occurred three times more frequently among maxillary molars than among mandibular molars. (3) Many molars with F.I. functioned well from 5 to 24 years. (4) Based on these findings, it is suggested that two aspects of molars with F.I. be reevaluated, i.e. their prognosis and treatment. Further it is suggested that therapy for these teeth be designed to improve their functional environment and that in many instances removal of root or bone or extraction of the tooth is not necessary. (5) F.I. was detected more frequently in maxillary molars by radiographic examination than by clinical examination. On the other hand F.I. was detected more frequently in mandibular molars by clinical examination than by radiographic examination.