Purpose: The complexity of molar furcation areas provides a challenge for periodontal treatment that leads to more tooth loss when compared to other tooth types. Furcation entrance dimension (FED) of molar teeth affects treatment outcomes and prognosis considerably. The aim of the present study was to determine the FED in first and second mandibular molars among Jordanian adults and to assess the relationship to gender. Materials and Methods: The sample consisted of 430 first and second mandibular molars that were extracted in four dental clinics of the Ministry of Health. The teeth were sorted according to gender after extraction and those meeting the following conditions were selected: intact crowns with completely preserved cementoenamel junction, intact furcation areas and roots lacking restorations. The FEDs were measured under magnification using a computer system (AutoCad 2005) equipped with a digital camera (HP Photosmart). Results: A high percentage (63.5%) of FEDs were wider than 0.75 mm, with buccal furcations wider than lingual ones and first molar furcations having wider furcations than those of second molars among Jordanian adults. A significant difference in the mean FEDs between male and female patients was observed in the second molar. Conclusions: Furcation involvements in Jordanian adults are more suitable for treatment with standard hand curettes than those reported in other cultures, as a high percentage (63.5%) of the FEDs were wider than the width of a standard Gracey curette blade. A significant difference between sexes in the second molar was observed. The use of other instruments, such as ultrasonic devices, should be taken into account during scaling and root planing as about 37.5% of the furcation would not be adequately instrumented with standard curettes. Keywords: curette, furcation, perio, plaque, root planing